Optical scanner and image forming apparatus

ABSTRACT

An optical scanner is capable of effectively correcting a difference with respect to the sub-scanning direction between the positions of optical spots for scanning photoconductor drums of a tandem type color image forming apparatus. The optical scanner includes an optical axis adjusting part. The optical axis adjusting part uses a deflecting mirror or a wedge-shaped prism to deflect the optical axis of an optical beam with respect to the sub-scanning direction. As a result, it is possible to accurately correct a resist difference among individual image forming stations and form a high-quality color image without any color displacement.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to an optical scanner andan image forming apparatus, and more particularly to an optical scannerthat can be preferably used as a writing system for a digital copier, alaser printer, a laser plotter and a facsimile, and an image formingapparatus that can be preferably used as a multi-color image formingapparatus for forming a color image by superposing color toner images.

[0003] 2. Description of the Related Art

[0004] In an image forming apparatus for forming an image in accordancewith Carlson process, a latent image forming process, a developingprocess and a transferring process are conducted in response to rotationof a photoconductor drum. If the photoconductor drum has an eccentricrotational axis or a drive motor for rotating the photoconductor drumhas speed variations, the latent image forming process through thetransferring process cannot be completed at a uniform time. As a result,a pitch irregularity, that is, an irregularity of intervals (scanningpitches) between individual optically-written line images, arises in thesub-scanning direction of a transferred image, resulting in a densityirregularity.

[0005] In a tandem type color image forming apparatus, a plurality ofphotoconductor drums are arranged along the shift direction of atransferred member. The tandem type color image forming apparatus formsa multi-color image or a full-color image by transferring andsuperposing different color images formed on the photoconductor drums onthe transferred member sequentially. If the pitch irregularity is causedin the tandem type color image forming apparatus, there is a risk thatthe individual superposed toner color images may be misaligned to eachother. In this case, a color displacement or a color change is generatedin the formed multi-color image or the formed full-color image,resulting in degradation of image quality.

[0006] In addition, a pitch irregularity arises in the transferring partbecause of variations of shift speeds of a transfer belt as atransferred member and a carrier belt to carry transferred paper. As aresult, the density irregularity, the color displacement and the colorchange are caused. This pitch irregularity is caused by eccentricity androtational speed variations of drive motors for rotating the transferbelt and the carrier belt.

[0007] In order to overcome problems such as the density irregularity,the color displacement and the color change, therefore, it is necessaryto eliminate rotational speed variations of a drive motor for driving aphotoconductor drum and overcome eccentricity and rotational speedvariations of a drive motor for driving the transfer belt or the carrierbelt. However, it is impossible to totally eliminate criteria of stablemachining and load changes of transmission systems. As a result, it isimpossible to totally eliminate the above-mentioned eccentricity and therotational speed variations.

[0008] In the tandem type color image forming apparatus, if opticalscanners fail to uniformly align resist positions of individual colorlatent images on the corresponding photbconductor drums with highaccuracy, there is a risk that the produced color image may includecolor displacement and color change. Additionally, when the opticalscanners write latent images by individual scanning lines, there isanother risk that the scanning lines may have different slopes from eachother. Furthermore, if the scanning lines are curved in degreesdifferent from each other, the color displacement and the color changeappears similarly.

[0009] Japanese Laid-Open Patent Application No. 08-014731 discloses amethod for eliminating influences due to a rotational irregularity of aphotoconductor drum. In this disclosed method, a time period from latentimage forming process to the transferring process is set as an integralmultiple of the period of the rotational irregularity. As a result,since the phase of a periodically varying position difference at time ofthe latent image formation coincides with the phase of a periodicallyvarying position difference at time of the transfer, it is possible tocancel the influences.

[0010] Japanese Laid-Open Patent Application No. 10-197810 discloses amethod for dynamically controlling an optical scanner. In this disclosedmethod, periodical emergence of the pitch irregularity is focused. Whenthe pitch irregularity is detected, a correction mirror is shiftedcorresponding to rotation of a photoconductor drum.

[0011] The pitch irregularity appears in an image as syntheses of lowfrequency factors and high frequency factors wherein the low frequencyfactors result from rotational irregularities of drive motors for aphotoconductor drum, a transfer belt and a carrier belt and, on theother hand, the high frequency factors result from engagement of gearsof transmission systems. As an image is required to have higher imagequality, more accurate gears are used for the transmission systems andthere is a stronger tendency that the photoconductor drum and thetransfer belt are directly driven by the drive motors so thatmalfunctions between the transmission systems cannot affect imagedegradation. Also, by increasing the inertial force by means of aflywheel, the high frequency factors can be reduced.

[0012] However, it is impossible to prevent the influences exerted bythe low frequency factors due to the load variations and others that areinvolved in the eccentricity and assembly differences in associationwith fabrication precision of parts thereof. Therefore, it is moreimportant to suppress the low frequency factors.

[0013] Especially in the tandem type color image forming apparatus, thepitch irregularity period with respect to the sub-scanning direction,which results from variations of transferring timings of toner images,has a different phase or a different amplitude for each image. Accordingto the above-mentioned methods, therefore, it is impossible to arrangedot positions of the individual images with high accuracy.

[0014] In order to adapt resist positions of individual latent images, adifference between the resist positions is detected through imagesrecorded on a transferred member. Additionally, the resist positionswith respect to the sub-scanning direction are aligned by adjustingwriting timings.

[0015] On the other hand, there are some correction methods forcorrecting the curvature and gradient of a scanning line. JapanesePatent No. 3049606 discloses a correction method for correcting thecurvature and gradient of a scanning line by curving a reflection mirrordisposed in an optical path and inclining the reflection mirror of asurface parallel to a transfer surface. Japanese Laid-Open PatentApplication No. 11-064758 discloses a correction method by changingheights of the optical axes of a part of lenses constituting an imageforming optical system. Japanese Laid-Open Patent Application No.10-268217 discloses a correction method by forcing the body of a lens tobe curved. Japanese Laid-Open Patent Application No. 11-153765 disclosesa correction method for rotating a part of lenses constituting an imageforming optical system in the directions of the optical axes of thelenses.

[0016] In recent years, a resin-molded lens has been used as an imageforming part of an optical scanner. Such a lens has some advantages inthat, for example, a resin-molded lens can be shaped to have complicatedsurface shape at a reasonable cost. In contrast, the lens hasdisadvantages in that even if the curvature and gradient of a scanningline is initially adjusted, harmful curvature and gradient may arisebecause of deformation of the lens body due to temperature variations ofthe environment.

[0017] In particular, when a resin-molded lens is disposed away from adeflecting part, the stiffness of the resin-molded lens tends to besmall because the lens is longer with respect to the main scanningdirection. As conventional methods, if one surface of the lens withrespect to the sub-scanning direction is in contact with something tomaintain the orientation thereof, there is a risk that the lens body maybe deformed because the lens is retained in a condition where stresssuch as a warp and a torsion is imposed on the lens.

[0018] If the curvature of a scanning line is attempted to be correctedby forcibly curving a resin-molded lens in accordance to JapaneseLaid-Open Patent Application No. 10-268217, there is a risk that thesurface shape of the lens is deformed by stress concentration. If anoptical beam is deflected toward the refractive index of interior of alens whose distribution is centered at the optical axis thereof withrespect to the sub-scanning direction in accordance with JapaneseLaid-Open Patent Application No. 11-064758, there is a risk that thediameter of a beam spot is not uniform on a photoconductor drum.

[0019] Additionally, a base member for retaining a resin-molded lens hasthermal conductivity different from the outer atmosphere, resulting in atemperature difference between the lens surface in contact with the basemember and the opposite lens surface exposed to the outer atmosphere. Asa result, there is a problem that the lens body is deformed and curvedover time.

[0020] In addition, a resist difference is conventionally detected basedon a resist difference detection pattern recorded on a transferred bodyand is adjusted with respect to the sub-scanning direction by changingthe writing timing as disclosed in Japanese Patents No. 3049606 and No.3078830.

[0021] Japanese Laid-Open Patent Application No. 11-064769 discloses amethod of correcting a scanning position by using a galvanometer mirror,which is disposed between an illuminant and an image forming opticalsystem, to incline the optical axis of an optical beam in thesub-scanning direction.

[0022] Japanese Patent No. 2672313 discloses a method of correcting ascanning position by parallel-shifting a folding mirror.

[0023] When a multi-color image forming apparatus, in which a pluralityof image forming stations are disposed along the carrying direction of atransferred member, forms a color image by superposing individual simplecolor images, color displacement or color change appears in the formedcolor image if resist positions, where individual latent images formedby the image forming stations are transferred, do not accuratelycoincide with each other.

[0024] However, even if an optical scanner is initially adjusted tocorrect a difference between scanning positions, which can cause adifference between resist positions, between individual image formingstations, there is a risk that temperature variations may deform thehousing of the optical scanner and cause variations of the refractiveindex of a scanning lens. Therefore, it is impossible to avoidvariations of resist positions over time.

[0025] For this reason, although the above-mentioned difference betweenresist positions is periodically detected and corrected, it isimpossible to align the resist positions at the writing head of an imageuniformly. As a result, for example, if the transferred member does notmove at a constant speed, a more significant color displacement andcolor change can appear at the writing end of the image.

[0026] A difference between resist positions is conventionally correctedby adjusting the writing timings of the individual image formingstations. A synchronizing detection signal of each surface of a polygonmirror is used as triggers to determine the writing timings. For thisreason, the writing timings cannot be adjusted for less than thesub-scanning pitch, which corresponds to the recording density. As aresult, there is a risk that a difference between the resist positions,whose maximum size is a half of the sub-scanning pitch, may appear on animage.

[0027] Additionally, if there is a speed difference between thetransferring position and the detecting position due to speed variationsof the transferred body, there is a risk that a detected resistdifference may contain an error. Furthermore, if the synchronizingdetection sensor is not accurately positioned due to thermal expansionof the housing, the synchronizing detection signal is generated atvarious timings. The detected resist difference and the synchronizingdetection signal is used as references of the feedback control of theoptical scanner. Thus, it is impossible to align the resist positionsuniformly based on such inaccurate references even if the adjustingprocess is properly operated.

[0028] In addition, a recent increase in the operational speed of colorimage forming apparatuses has realized practical use of color digitalcopiers, color laser printers and so on. In a four-drum tandem typecolor image forming apparatus, for example, four photoconductor drumsare arranged in the carrying direction of record papers. A plurality ofoptical scanning systems corresponding to the individual photoconductordrums simultaneously expose the photoconductor drums so as to formlatent images. These latent images are made visible by using differentcolor developers such as yellow, magenta, cyan and black. Then, thesedeveloped simple color images are sequentially superposed andtransferred onto a same record paper so as to form a full-color image.

[0029] Alternatively, a one-drum type image forming apparatus has onlyone photoconductor drum. In such a one-drum type image formingapparatus, the photoconductor drum is rotated as many times as thenumber of prepared colors. For each rotation, a latent image formingprocess (exposing process), a developing process and a transferringprocess are performed for the photoconductor drum, and then theresulting visible simple color images are superposed and transferredonto a same record paper so as to form a full-color image. As anotherembodiment of the one-drum type color image forming apparatus, after thevisible simple color images are formed, the visible simple color imagesmay be temporarily superposed onto an intermediate transferred memberand then transferred onto the record paper.

[0030] The four-drum tandem type color image forming apparatus has anadvantage compared to the one-drum type color image forming apparatus inthat the four-drum tandem type color image forming apparatus can producea color image and a monochrome image at a same speed. Thus, thefour-drum tandem type color image forming apparatus is more suitable tohigh speed printing. In contrast, since the four-drum tandem type colorimage forming apparatus contains four optical scanning systems to exposefour photoconductor drums, the four-drum tandem type color image formingapparatus tends to have a greater size and it is necessary to reduce thesize thereof. Additionally, the four-drum tandem type color imageforming apparatus has another problem in that color displacement mayoccur when individual color toner images corresponding to the fourphotoconductor drums are superposed and transferred onto a record paper.

[0031] In particular, the color displacement can be caused with respectto the sub-scanning direction by the following factors.

[0032] A speed variation of a photoconductor with respect to thecircumferential direction (sub-scanning direction).

[0033] A speed variation of an intermediate transferred member withrespect to the circumferential direction (sub-scanning direction).

[0034] A position difference between photoconductors.

[0035] A position difference of optical spots between optical scanningsystems.

[0036] If a plurality of optical beams are simultaneously used to writelatent images on individual photoconductor drums, there is a risk thatmisalignment with respect to the sub-scanning direction may be causedcorresponding to the number of prepared optical beams because a polygonscanner is not rotated synchronously with the photoconductor drums ingeneral.

[0037] The following conventional methods for suppressing such colordisplacement are presented.

[0038] Japanese Laid-Open Patent Application No. 2001-133718 disclosesan invention that can make scanning lines on individual photoconductordrums coincide with each other by adjusting positions of the individualscanning means or the housings thereof relative to the photoconductordrums. According to this invention, however, the adjustment mechanismbecomes complicated and it takes a large amount of adjustment time. Inaddition, since the heavy housings are adjusted, it is difficult tocorrespond to changes over time due to temperature variations. Also, itis difficult to accurately correct color displacement during printingoperation or color displacement due to variations of the environment.

[0039] Japanese Laid-Open Patent Application No. 2001-100127 discloses amethod for controlling the position of an optical beam with respect tothe sub-scanning direction by using a galvanometer mirror. According tothis disclosed method, however, since the galvanometer mirror is toosensitive for the purpose of controlling the optical beam position ofwith respect to the sub-scanning direction, the galvanometer mirror ishighly influenced by external vibrations. In order to obtain a betterbeam spot diameter, it is necessary to satisfy high surface accuracy(about four times of a transmission surface).

[0040] Japanese Laid-Open Patent Application No. 10-239939 discloses acolor image forming apparatus that includes color displacementcorrection means. In this color image forming apparatus, an optical beamfor first writing an image on a photoconductor is selected among aplurality of optical beams based on a phase relation between a referenceintermediate transferring signal and a line synchronizing signal so asto adjust starting positions for writing individual color images withrespect to the sub-scanning direction. According to this color imageforming apparatus, however, it is difficult to correct colordisplacement smaller than one line. For instance, if individual simplecolor images are written at 600 dpi (dots per inch) there is a risk thata full-color image generated from the individual color images may havecolor displacement of at least more than 42 μm.

SUMMARY OF THE INVENTION

[0041] It is a general object of the present invention to provide anoptical scanner and an image forming apparatus in which theabove-mentioned problems are eliminated.

[0042] A first specific object of the present invention is to provide anoptical scanner and an image forming apparatus that can realize asatisfactory color image without any color displacement and color changeby effectively correcting and lowering a pitch irregularity due to thelow frequency factors, more specifically, by effectively correctingperiodic pitch irregularities caused in individual color image formingstations and effectively lowering harmful the curvature and gradient ofa scanning line generated by a resin-molded lens serving as an imageforming system.

[0043] A second more specific object of the present invention is toprovide an optical scanner and an image forming apparatus that cancorrect a difference between resist positions of individual imageforming stations with high accuracy and form a satisfactory color imagewithout any color displacement and color change due to variations overtime, especially, temperature variations over time.

[0044] A third more specific object of the present invention is toprovide an optical scanner and an image forming apparatus that caneffectively correct color displacement among individual colors and forma satisfactory color image even if scanning positions are misalignedwith respect to the sub-scanning direction due to rapid temperaturevariations and others.

[0045] A fourth more specific object of the present invention is toprovide an optical scanner and an image forming apparatus that caneffectively correct and reduce a pitch irregularity due to theabove-mentioned low frequency factors and can form a satisfactory colorimage without any color displacement and color change by effectivelycorrecting a periodic pitch irregularity generated by individual imageforming stations and effectively reducing the curvature and gradient ofa scanning line generated by a resin-molded lens serving as an imageforming system.

[0046] In order to achieve the above-mentioned objects, there isprovided according to one aspect of the present invention an opticalscanner for scanning an image carrier, including: an illuminant partemitting an optical beam; a deflecting part deflecting the optical beam;an image forming part focusing the deflected optical beam on the imagecarrier; and an optical axis adjusting part being provided between theilluminant part and the deflecting part, the optical axis adjusting partadjusting a beam spot position of the optical beam on the image carrierwith respect to a sub-scanning direction.

[0047] In the above-mentioned optical scanner, the optical axisadjusting part may include a movable mirror.

[0048] In the above-mentioned optical scanner, the movable mirror mayhave at least one vibration mode in which the movable mirror vibrateswith respect to the sub-scanning direction.

[0049] In the above-mentioned optical scanner, the optical axisadjusting part may slightly vibrate the beam spot position of theoptical beam on the image carrier with respect to the sub-scanningdirection slowly relative to a period of optical scanning.

[0050] In the above-mentioned optical scanner, the optical axisadjusting part may include a phase adjusting part adjusting a phase ofvibration.

[0051] In the-above-mentioned optical scanner, the optical axisadjusting part may include an amplitude adjusting part adjusting anamplitude of vibration.

[0052] Additionally, there is provided according to another aspect ofthe present invention an image forming apparatus, including: a pluralityof image carriers; an optical scanner forming latent images on theplurality of image carriers, including: an illuminant part including aplurality of illuminants, the plurality of illuminants emitting opticalbeams; a deflecting part deflecting the optical beams; an image formingpart focusing the deflected optical beams on the plurality of imagecarriers; and an optical axis adjusting part being provided between theilluminant part and the deflecting part, the optical axis adjusting partadjusting beam spot positions of the optical beams on the plurality ofimage carriers with respect to a sub-scanning direction; a developingpart developing the latent images so as to form visible images; and atransferred member onto which the visible images are transferred fromthe plurality of image carriers, wherein the optical axis adjusting partincludes a movable mirror and slightly vibrates the beam spot positionsof the optical beams on the image. carriers with respect to thesub-scanning direction slowly relative to a period of optical scanning.

[0053] In the above-mentioned image forming apparatus, the optical axisadjusting part may slightly adjust rotational time of the image carriersfrom writing positions onto the image carriers to a transferringposition onto the transferred member while the latent images are formedon the plurality of image carriers.

[0054] In the above-mentioned image forming apparatus, the illuminantpart may include a selecting part selecting one of the plurality ofilluminants for an optical beam that optically scans a head line withrespect to the sub-scanning direction from which the latent images areformed corresponding to the rotational time of the image carriers fromthe writing positions onto the image carriers to the transferringposition onto the transferred member.

[0055] According to the above-mentioned inventions, it is possible toeffectively correct periodic variations of a pitch of scanning lines dueto low-frequency factors, a difference between resist positions forforming a color image,. and the slope and curvature of a scanning line.In this fashion, since the optical scanner can properly write an imageon an image carrier, the image forming apparatus is capable of form ahigh-quality color image by effectively suppressing density unevennessdue to the pitch irregularity of scanning lines and reducing colordisplacement and color change.

[0056] Furthermore, in the above-mentioned optical scanner, the opticalaxis adjusting part may include a wedge-shaped prism.

[0057] In the above-mentioned optical scanner, the optical axisadjusting part may adjust the beam spot position of the optical beamwith respect to the sub-scanning direction by rotating the wedge-shapedprism approximately on an optical axis.

[0058] In the above-mentioned optical scanner, the optical axisadjusting part may adjust the beam spot position of the optical beamduring writing of an image.

[0059] Additionally, there is provided according to another aspect ofthe present invention an image forming apparatus, including: a pluralityof image carriers; an optical scanner forming latent images on theplurality of image carriers, said optical scanner including: anilluminant part including a plurality of illuminants, the plurality ofilluminants emitting optical beams; a deflecting part deflecting theoptical beams; an image forming part focusing the deflected opticalbeams on the plurality of image carriers; and an optical axis adjustingpart being provided between the illuminant part and the deflecting part,the optical axis adjusting part adjusting beam spot positions of theoptical beams on the plurality of image carriers with respect to asub-scanning direction; a developing part developing the latent imagesso as to form visible images; and a transferred member onto which thevisible images are transferred from the plurality of image carriers,wherein the optical axis adjusting part comprises a wedge-shaped prismand adjusts the beam spot positions of the optical beams with respect tothe sub-scanning direction by rotating the wedge-shaped prismapproximately on an optical axis.

[0060] The above-mentioned image forming apparatus may further include aposition difference detecting part detecting a difference between thebeam spot positions of the optical beams on the plurality of imagecarriers with respect to the sub-scanning direction.

[0061] In the above-mentioned image forming apparatus, the optical axisadjusting part may adjust the beam spot positions of the optical beamwith respect to the sub-scanning direction based on the differencedetected by the position difference detecting part during writing of animage.

[0062] According to the above-mentioned inventions, since thewedge-shaped prism can be driven under simple mechanism and means, it ispossible to correct misalignment of a scanning position and realize ahighly accurate scanning position.

[0063] Additionally, according to the above-mentioned inventions, evenif the position of an optical spot is drastically varied, it is possibleto correct the position of the optical spot.

[0064] Furthermore, there is provided according to another aspect of thepresent invention an optical scanner for scanning an image carrier,including: an illuminant part emitting an optical beam; a deflectingpart deflecting the optical beam; an image forming part focusing thedeflected optical beam on the image carrier; and an optical axisadjusting part being provided between the illuminant part and thedeflecting part, the optical axis adjusting part adjusting a writingstart position of the optical beam on the image carrier with respect toa sub-scanning direction.

[0065] In the above-mentioned optical scanner, the optical axisadjusting part may include a movable mirror.

[0066] The above-mentioned optical scanner may further include anoptical beam detecting part detecting a position of the optical beamwith respect to a main scanning direction.

[0067] The above-mentioned optical scanner may further include a housingintegrally accommodating the illuminant part, the deflecting part, theimage forming part, and the optical beam detecting part.

[0068] In the above-mentioned optical scanner, the optical beamdetecting part may be disposed at a position in the housing toward amain scanning end from a position detected by the optical beam detectingpart and the detected position may be allowed to conduct free expansionrelative to the position of the optical beam detecting part.

[0069] Additionally, there is provided according to another aspect ofthe present invention an image forming apparatus, including: a pluralityof image carriers; an optical scanner forming latent images on theplurality of image carriers, the optical scanner including: anilluminant part including a plurality of illuminants, the plurality ofilluminants emitting optical beams; a deflecting part deflecting theoptical beams; an image forming part focusing the deflected opticalbeams on the plurality of image carriers; and an optical axis adjustingpart being provided between the illuminant part and the deflecting part,the optical axis adjusting, part adjusting writing start positions ofthe optical beams on the plurality of image carriers with respect to asub-scanning direction; a developing part developing the latent imagesso as to form visible images; and a transferred member onto which thevisible images are transferred from the plurality of image carriers,wherein the optical axis adjusting part comprises a movable mirror andadjusts the writing start positions of the optical beams on theplurality of image carriers with respect to the sub-scanning direction.

[0070] The above-mentioned image forming apparatus may further include aresist position difference detecting part detecting a difference betweenthe writing start positions of the optical beams on the plurality ofimage carriers with respect to the sub-scanning direction.

[0071] In the above-mentioned image forming apparatus, the optical axisadjusting part may be controlled through feedback based on thedifference between the writing start positions detected by the resistposition difference detecting part so as to adjust the writing startpositions of the optical beams.

[0072] In the above-mentioned image forming apparatus, a distancebetween each of transferring positions of the latent images onto thetransferred member and a detecting position of the resist positiondifference detecting part may be set as an approximately integermultiple of a circumferential length of a driving roller for driving thetransferred member.

[0073] According to the above-mentioned inventions, although aconventional image forming apparatus align resist positions ofindividual image forming stations per one line corresponding to therecording density, the image forming apparatus according to the presentinvention is capable to aligning the resist positions with higheraccuracy. As a result, it is possible to form a high-quality color imagewithout any color displacement and color change.

[0074] Additionally, according to the above-mentioned inventions, evenif the transfer belt moves at variable speeds, it is possible to makethe circumferential speed of the transfer belt at a transferringposition equal to the circumferential speed at a detecting position. Asa result, it is possible to detect a difference between resist positionsat the transferring position with high accuracy. Through the detectionimprovement, the image forming apparatus is capable of forming ahigh-quality color image.

[0075] Additionally, according to the above-mentioned inventions, evenif the temperature is varied, a synchronization detecting position isstably maintained. As a result, it is possible to effectively controlthe phases of writing timings between individual image forming stationsand form a high-quality color image without any color displacement andcolor change.

[0076] In the above-mentioned optical scanner, the optical axisadjusting part may include a wedge-shaped prism.

[0077] In the above-mentioned optical scanner, the optical axisadjusting part may adjust the writing start position of the optical beamwith respect to the sub-scanning direction by rotating the wedge-shapedprism approximately on an optical axis.

[0078] Additionally, there is provided according to another aspect ofthe present invention an image forming apparatus, including: a pluralityof image carriers; an optical scanner forming latent images on theplurality of image carriers, the optical scanner including: anilluminant part including a plurality of illuminants, the plurality ofilluminants emitting optical beams; a deflecting part deflecting theoptical beams; an image forming part focusing the deflected opticalbeams on the plurality of image carriers; and an optical axis adjustingpart being provided between the illuminant part and the deflecting part,the optical axis adjusting part adjusting writing start positions of theoptical beams on the plurality of image carriers with respect to asub-scanning direction; a developing part developing the latent imagesso as to form visible images; and a transferred member onto which thevisible images are transferred from the plurality of image carriers,wherein the optical axis adjusting part comprises a wedge-shaped prismand adjusts the writing start positions of the optical beams withrespect to the sub-scanning direction by rotating the wedge-shaped prismapproximately on an optical axis.

[0079] The above-mentioned image forming apparatus may further include aresist position difference detecting part detecting a difference betweenthe writing start positions of the optical beams on the plurality ofimage carriers with respect to the sub-scanning direction.

[0080] In the above-mentioned image forming apparatus, the optical axisadjusting part may be controlled through feedback based on thedifference between the writing start positions detected by the resistposition difference detecting part so as to adjust the writing startpositions of the optical beams.

[0081] In the above-mentioned image forming apparatus, the plurality ofimage carriers may include just four image carriers corresponding tofour colors: black, yellow, magenta and cyan, that are arranged in atandem fashion, one of the four colors may be predetermined as areference color, and the optical axis adjusting part may have threewedge shaped prisms for adjusting writing start positions of opticalbeams for scanning three image carriers other than the reference colorsuch that the writing start positions coincide with a writing startposition of an optical beam for scanning an image carrier for thereference color.

[0082] According to the above-mentioned inventions, even if a scanningposition is misaligned with respect to the sub-scanning direction due todrastic temperature variations and others, it is possible to effectivelycorrect a color difference between individual colors and form a propercolor image.

[0083] Additionally, according to the above-mentioned inventions, evenif a scanning position is greatly misaligned to the scanning position ofa reference color, it is possible to easily correct a color differencethereof because the number of adjusting portions and an adjusted amountare reduced.

[0084] Additionally, according to the above-mentioned inventions, evenif the writing start position on an image carrier is misaligned overtime, it is possible to correct the misalignment of the writing startposition with respect to the sub-scanning direction.

[0085] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0086]FIG. 1 is a diagram illustrating an exemplary optical scanneraccording to the present invention;

[0087]FIG. 2 is a diagram illustrating an image forming part of anexemplary tandem type color image forming apparatus according to thepresent invention;

[0088]FIG. 3 is an exploded perspective view of an exemplary movablemirror module serving as an optical axis adjusting part according to thepresent invention;

[0089]FIGS. 4A and 4B are diagrams illustrating variations of theoptical axis adjusting part according to the present invention;

[0090]FIG. 5 shows an exemplary relation between a writing position ontoa photoconductor drum and a transferring position;

[0091]FIG. 6 shows an exemplary placement of optical spots emitted froman illuminant unit according to one embodiment of the present invention;

[0092]FIG. 7 is a block diagram illustrating a circuit for selecting anilluminant in a semiconductor array;

[0093]FIGS. 8A through 8C are diagrams illustrating exemplary variationsof a recording pitch with respect to a sub-scanning direction;

[0094]FIG. 9 is a block diagram illustrating a drive circuit for drivinga movable mirror;

[0095]FIG. 10 is a diagram for explaining slope of a scanning line;

[0096]FIGS. 11A and 11B are diagrams for explaining curvature of ascanning line;

[0097]FIGS. 12A and 12B are diagrams illustrating a supported toroidallens;

[0098]FIG. 13 is a diagram illustrating the structure of an exemplaryimage forming apparatus according to the present invention;

[0099]FIG. 14 is a diagram for explaining a sensor part for detectingthe position of an optical beam with respect to the sub-scanningdirection;

[0100]FIG. 15 is a diagram for explaining a curvature of the bottomsurface of a housing for an optical scanner according to one embodimentof the present invention;

[0101]FIG. 16 is a diagram illustrating the structure of an imageforming apparatus according to another embodiment of the presentinvention;

[0102]FIG. 17 is a diagram illustrating the structure of an imageforming part of the image forming apparatus according to the embodiment;

[0103]FIG. 18 is a diagram illustrating the structure of a portion of anoptical scanner according to one embodiment of the present invention;

[0104]FIG. 19 is a cross-sectional view of a synchronization detectingsensor and an end detecting sensor according to one embodiment of thepresent invention;

[0105]FIGS. 20A and 20B are cross-sectional views of an exemplaryhorizontally installed optical detecting part and an exemplaryvertically installed optical detecting part, respectively, according tothe present invention;

[0106]FIG. 21 is a cross-sectional view of an exemplary movable mirrormodule as an optical axis adjusting part according to the presentinvention;

[0107]FIG. 22 is a diagram illustrating a writing position and atransferring position on a photoconductor drum;

[0108]FIG. 23 is a diagram illustrating an arrangement of beam spots ona photoconductor drum;

[0109]FIG. 24 is a block diagram of a control mechanism for correcting adifference of writing start timings;

[0110]FIG. 25 is a block diagram of a drive circuit for driving amovable mirror;

[0111]FIGS. 26A through 26C are diagram illustrating exemplaryvariations of a recording pitch with respect to a sub-scanningdirection;,

[0112]FIG. 27 is an exploded perspective view of a correction mechanismfor correcting slope and curvature of a scanning line;

[0113]FIG. 28 is a diagram illustrating the structure of a variation ofthe image forming part according to the present invention;

[0114]FIG. 29 is a diagram illustrating the structure of a variation ofthe optical axis adjusting part according to the present invention;

[0115]FIG. 30 is a diagram illustrating the structure of an imageforming part of an image forming apparatus according to one embodimentof the present invention;

[0116]FIG. 31 is a diagram for explaining a correction mechanism forcorrecting the position of an optical spot with respect to thesub-scanning direction by means of a wedge-shaped prism;

[0117]FIG. 32 is a diagram illustrating the structure of a lead screwtype actuator serving as a writing start position correcting part;

[0118]FIG. 33 is a diagram for explaining a detection mechanism by meansof non-parallel photodiode sensors serving as a beam spot positiondetecting part;

[0119]FIG. 34 is a graph of temperature changes in an optical scanner atsuccessive printing;

[0120]FIG. 35 is a diagram illustrating the structure of anotherexemplary image forming apparatus according to the present invention;

[0121]FIG. 36A is a diagram for explaining position differences of anoptical spots with respect to the sub-scanning direction due to a speedvariation of an intermediate transferred body;

[0122]FIG. 36B is a diagram for explaining positions differences of anoptical spot with respect to the sub-scanning direction after theposition of the optical spot is corrected; and

[0123]FIG. 37 is a graph of a relation between drive frequency of astepping motor and torque.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0124] In the following, embodiments of the present invention will bedescribed with reference to the accompanying drawings.

[0125]FIG. 1 shows the structure of an optical scanner according to oneembodiment of the present invention, and FIG. 2 shows the structure ofan image forming part of a tandem type color image forming apparatusaccording to one embodiment according to the present invention. Theoptical scanner in FIG. 1 constitutes the image forming part of thetandem type color image forming apparatus in FIG. 2. The optical scanneroptically scans one photoconductor drum included in the tandem typecolor image forming apparatus. In order to avoid confusion, parts inFIG. 2 are designated by reference numerals different from parts in FIG.1 even if some of the parts are identical objects.

[0126] In the image forming part of the tandem type color image formingapparatus in FIG. 2, four photoconductor drums 121, 122, 123 and 124 arearranged along the shift direction of the circumferential surface of atransfer belt 125, which serves as a transferred member. Afterelectrostatic latent images are optically written and formed on theindividual photoconductor drums 121, 122, 123 and 124, the electrostaticlatent images are made visible with distinct color toners. The colortoner images are sequentially superposed and transferred on the transferbelt 125 to form a color image. The color image is transferred and fusedon (not illustrated) a recording medium, for example, a record paper soas to form a color image.

[0127] The optical scanners, each of which optically scans respectivephotoconductor drums 121, 122, 123 and 124, share a polygon mirror 126,which serves as a deflecting part. Also, the tandem type color imageforming apparatus contains lenses, which serve as image forming parts,for focusing optical beams deflected by the polygon mirror 126 on thecorresponding photoconductor drums 121, 122, 123 and 124. Some of thelenses are also shared for the photoconductor drums 121, 122, 123 and124.

[0128] Each of illuminant units 127 and 128 includes a pair ofsemiconductor lasers for emitting optical beams, which are deflected onsame deflection reflection surfaces of the polygon mirror 126. Thetandem type color image forming apparatus in FIG. 2 will be describedlater in detail.

[0129] Referring to FIG. 1, for example, the optical, scanner in FIG. 1corresponds to the optical scanner that optically scans thephotoconductor drum 122 with an optical beam emitted by the illuminantunit 127 in the configuration of the tandem type color image formingapparatus in FIG. 2.

[0130] In the optical scanner in FIG. 1, semiconductor lasers 101 and102 emit optical beams for optically scanning a photoconductor drum 111and (not illustrated) another photoconductor drum, respectively. The twooptical beams are converted into parallel luminous fluxes throughcoupling lenses 103 and 104, respectively, and then enter a synthesizingprism 105. The synthesizing prism 105 is formed as a combination of aparallelogram prism and a trapezoid prism.

[0131] The optical beam from the semiconductor laser 101 is transmittedthrough the trapezoid prism of the synthesizing prism 105. On the otherhand, the optical beam from the semiconductor laser 102 is reflected ona pair of parallel reflection surfaces of the parallelogram prism of thesynthesizing prism 105 sequentially and then exits close to the opticalbeam from the semiconductor laser 101 such that the two optical beamshave a predetermined interval each other with respect to thesub-scanning direction as illustrated in FIG. 1.

[0132] The two optical beams pass through positions away from the centeraxis of the cylindrical lens 106 with respect to the sub-scanningdirection. Then, the optical beams from the semiconductor lasers 101 and102 are reflected on respective movable mirrors 107 and 108 so as tocross each other on a deflection reflection surface of a polygon mirror100 with respect to the sub-scanning direction. The reflected opticalbeams are focused near the deflection reflection surface of the polygonmirror 100 as a line beam with respect to the main scanning direction.Through rotation of the rotationally driven polygon mirror 100, the linebeam becomes a luminous flux deflected at a constant angular velocity.Hereinafter, only the optical beam propagating to the photoconductordrum 111 will be described.

[0133] After the optical beam is deflected by the polygon mirror 100,the deflected optical beam is transmitted through an fθ lens 109 and atoroidal lens. 110, each of which serves as the image forming part, andthen is focused on the circumferential surface of the photoconductordrum 111 as an optical spot to optically scan the circumferentialsurface.

[0134] In this embodiment, the fθ lens 109 and the toroidal lens 110extend in the sub-scanning direction.

[0135] The movable mirror 107 is capable of deflecting the optical beamto the photoconductor drum 111 with respect to the sub-scanningdirection and serves as an optical axis adjusting part for deflectingthe optical beam with respect to the sub-scanning direction.

[0136] The semiconductor lasers 101 and 102 comprise semiconductor laserarrays each of which is formed in a monolithic structure by arranging aplurality of illuminants at a pitch of slightly more than 10 μm as anarray. A compound image formation magnification of the coupling lens103, the cylindrical lens 106, the fθ lens 109 and the toroidal lens 110with respect to the sub-scanning direction is notated as β, and thepitch of the illuminants is notated as d. Then, if the optical scanneris designed to meet the equation β=p/d, optical spots from theilluminants are formed on the photoconductor drum 111 to have a pixelpitch p corresponding to a recording density.

[0137] Although the optically scanning process follows a “multi-beamscanning fashion”, it is possible to select one of the plurality ofilluminants to write the head line.

[0138] As previously mentioned, the two optical beams from thesemiconductor lasers 101 and 102 are deflected to cross each other nearthe polygon mirror 100 with respect to the sub-scanning direction. Inthis case, it is possible to separate the two optical beams after thedeflection by the polygon mirror 100 with respect to the sub-scanningdirection so that each of the deflected optical beams can travel towarda photoconductor drum different from each other. In the optical scannershown in FIG. 1, after the optical beam from the semiconductor laser 101is deflected by the polygon mirror 100, the deflected optical beam isguided toward the photoconductor drum 111 by two folding mirrors 112 and114.

[0139] In FIG. 1, a synchronization detecting sensor 113 takes a starttiming for writing in the main scanning direction.

[0140] It is noted that an optical path of the optical beam from thesemiconductor laser 102 is omitted in FIG. 1. In this embodiment, themovable mirrors 107 and 108 are configured as one movable mirror module.However, the movable mirrors 107 and 108 may be separately provided foreach of the two optical beams.

[0141]FIG. 3 is an exploded perspective view of the movable mirrormodule 160, which serves as the optical axis adjusting part according tothe present invention, accommodating the movable mirrors 107 and 108 inFIG. 1 wherein the movable mirrors 107 and 108 are designated by thereference numerals 162 and 163, respectively.

[0142] In the movable mirror module 160, first and second movablemirrors 162 and 163 are disposed in parallel to the sub-scanningdirection on a support substrate 161 shaped of a sintered metal. Theinterior of the movable mirror module 160 is sealed with a cap-shapedcover 165. An optical beam passes through a glass window 164 that isprovided at an aperture of the cover 165. Inactive gas is enclosed inthe sealed interior.

[0143] A movable mirror substrate is configured by joining an Sisubstrate 166 and an Si support frame 167 via an insulation film. The Sisubstrate 166 is formed of an Si substrate of 60 μm in thickness. Anetching process is performed to cut off portions of the first and thesecond movable mirrors 162 and 163 as well as the periphery of twistedbeams 168 and 169 to support the movable mirrors 162 and 163. Themovable mirrors 162 and 163 are coupled to a fixing frame 170 via thetwisted beams 168 and 169. Then, the movable mirrors 162 and 163 areprovided to be connected to the fixing frame 170 via the twisted beams168 and 169. The first and the second twisted beams 168 and 169 arearranged in parallel to the main scanning direction.

[0144] On the surface of the Si substrate 166, the movable mirrors 162and 163 have tooth-shaped ends at the sides orthogonal to the twistedbeams 168 and 169 and the fixing frame 170 has tooth-shaped ends, asillustrated in FIG. 3, so as to engage with each other. A metal film ofAu and others is deposited on the tooth-shaped portions of the movablemirrors 162 and 163 together with the tooth-shaped portion of the fixingframe 170. The tooth-shaped portions of the movable mirrors 162 and 163serve as first and second movable electrodes, and the tooth-shapedportion of the fixing frame 170 serves as first and second fixedelectrodes opposite to the first and the second movable electrodes.

[0145] When a voltage is applied to the first and the second fixedelectrodes, electrostatic force is generated between the mutually facingmovable electrodes. Consequently, the twisted beams 168 and 169 aretwisted and the movable mirrors 162 and 163 are separately rotated bysmall angles. If a pulse voltage is periodically applied to the firstand the second fixed electrodes, it is possible to cause the movablemirrors 162 and 163 to oscillate.

[0146] Here, if the twisted beams 168 and 169 are configured to havewidths and lengths corresponding to resonance frequency peculiar to theoscillating portions thereof, it is possible to make the amplitudelarger due to the oscillation. As a result, since the movable mirrors162 and 163 can be driven by applying weak intensity of electriccurrent, it is possible to reduce an amount of the power consumption. Inorder to prevent resonance caused by vibration transmission due torotation of photoconductor drums and others, however, it is preferablethat the resonance frequency be set as a sufficiently high resonancefrequency (more than 200 Hz) toward the vibration due to the rotation.

[0147] On the other hand, the resonance frequency may be set to correctpitch variations (banding) involved in the vibration transmission withrespect to the sub-scanning direction.

[0148] When the fixed and the movable electrodes are tooth-shaped asmentioned above, it is possible to increase the outer circumferentiallengths thereof as much as possible and make the lengths of theelectrodes greater. As a result, it is possible to obtain a greaterelectrostatic torque at a low voltage.

[0149] The Si support frame 167 is formed of an Si substrate of 200 μmin thickness, and an aperture is provided at the center of the Sisupport frame 167. The Si support frame 167 fixes a support portion ofthe Si substrate 166 so that the aperture can provide an oscillationarea of the movable mirrors 162 and 163. Lead terminals 172 are providedto pierce a support substrate 161 via an insulator. A wire bondingprocess is conducted for each of the lead terminals 172 such that eachof the fixed electrodes, each of pad parts 173, and each upper end ofthe lead terminals 172 are connected each other via the lead terminal172. As a result, it is possible to connect between the interior and theexterior of the packed electric wiring.

[0150] The cover 165 is embedded in a step part formed in the outercircumference of the support substrate 161, and the glass window 164 isjoined at the aperture from the inner side of the cover 165.

[0151] In this configuration, it is assumed that each of the movablemirrors 162 and 163 has 2 a in length, 2 b in width and d in thicknessand that each of the twisted beams 168 and 169 has L in length and c inwidth. Also, the density of the Si is notated as ρ and the materialconstant is notated as G. Then, the moment of inertia I of the movablemirrors 162 and 163 and the spring constant K of the twisted beams 168and 169 are given as follows;

I=(4abρd/3)·a ², and

K=(G/2L)·{cd(c ² +d ²)/12}.

[0152] Accordingly, the resonance frequency f is given as follows;

f=(1/2π)·(K/I)^(1/2)=(1/2π)·{Gcd(c ² +d ²)/24LI} ^(1/2).

[0153] Additionally, since the length L of the twisted beams 168 and 169is in proportion to an oscillation angle θ, the oscillation angle θ isrepresented as follows;

θ=(A/I)f ²,

[0154] where A is a constant. As seen from the above formula, theoscillation angle θ is in inverse proportion to the moment of inertia I.

[0155] In a case where the resonance frequency f is high, unless themoment of inertia I is reduced, the above formula says that theoscillation angle θ becomes small. In order to overcome this problem,the movable mirrors 162 and 163 according to this embodiment is designedto reduce the weight thereof by partially cutting off the back sidesthereof. As a result, it is possible to decrease the moment of inertiaI.

[0156] Also, electrostatic force F between a fixed electrode and amovable electrode is given as follows;

F=εHV ²/2δ,

[0157] where ε is a dielectric constant of air, H is the length of theelectrodes, V is an applied voltage and δ is the distance between thefixed electrode and the movable electrode.

[0158] Accordingly, the oscillation angle θ can be represented asfollows;

θ=B·F/I,

[0159] where B is a constant.

[0160] This formula asserts that the larger electrode length H is, thegreater oscillation angle θ is. In response to this fact, the fixedelectrode and the movable electrode are tooth-shaped in this embodimentof the present invention so as to obtain the large electrode length H.As a result, if the number of the teeth is n, it is possible to obtain a2n amount of drive torque.

[0161] Here, the viscous resistance P of air toward the movable mirroris given as follows;

P=C·ην ² ·E ³,

[0162] where C is a constant, η is the air density in the vicinity ofthe movable mirror, ν is the speed of the movable mirror, and E is thearea of the movable mirror.

[0163] In this embodiment, the movable mirrors are sealed, anddecompressed inactive gas is enclosed in the sealed interior aspreviously mentioned. As a result, it is possible to not only reduce theviscous resistance but also prevent changes of properties of the metalfilms on the movable mirrors due to chemical changes thereof.

[0164]FIG. 4A shows an exemplary movable mirror according to anotherembodiment of the present invention. This movable mirror has twooscillation modes.

[0165] In the movable mirror in FIG. 4A, an etching process is conductedfor an Si substrate 186. In the etching process, a movable mirror 181, afirst twisted beam 182 to support the movable mirror 181, a movableframe 184 connected to the first twisted beam 182, a second twisted beam183 to connect between the movable frame 184 and a fixing frame 185, andthe fixing frame 185 are formed of the Si substrate 186 as illustrated.

[0166] As shown in FIG. 4A, the movable mirror 181, the movable frame184 and the fixing frame 185 have tooth-shaped portions such that thetwisted beams 182 and 183 are sandwiched. In this configuration, themovable mirror 181, the movable frame 184 and the fixing frame 185 canengage with each other. The tooth-shaped portions of the movable mirror181, the movable frame 184 and the fixing frame 185 are deposited withmetal films of Au and others. Then, the tooth-shaped portions of themovable mirror 181 are set as a first movable electrode, thecorresponding tooth-shaped portions of the movable frame 184 are set asa first fixed electrode, the other tooth-shaped portions of the movableframe 184 are set as a second movable electrode, and the correspondingtooth-shaped portions of the fixing frame 185 are set as a second fixedelectrode.

[0167] When a voltage is applied to the first and the second fixedelectrodes, electrostatic force is generated between the fixedelectrodes and the corresponding movable electrodes. Since the twistedbeams 182 and 183 are twisted by the generated electrostatic force, itis possible to rotate the movable mirror 181 and the movable frame 184separately by small angles. If a pulse voltage is periodically appliedto the first and the second fixed electrodes, it is possible tooscillate the movable mirror 181 and the movable frame 184.

[0168] In this embodiment, the twisted beams 182 and 183 are disposed onthe same axis. Accordingly, if voltages of different frequencies areapplied to the first and the second fixed electrodes, it is possible tooscillate the movable mirror 181 at such an amplitude that twooscillation modes are overlapped in the same direction.

[0169] Similarly to the movable mirrors 162 and 163 in FIG. 3, themovable mirror 181 in FIG. 4A is disposed on the Si support frame suchthat the movable mirror 181 is sealed by the support substrate and thecover and inactive gas is enclosed in the decompressed interior thereof.Also, the electric wiring is provided to the movable mirror 181 like themovable mirrors 162 and 163.

[0170] In the above-mentioned implementation, the movable mirror isdriven by electrostatic force. However, the movable mirror may be drivenby a piezoelectric element and others.

[0171] The optical axis adjusting part is not limited to theabove-mentioned movable mirrors. Suitable means for changing the opticalaxis of an optical beam may be used as the optical axis adjusting part.As shown in FIG. 4B, for instance, it is possible to adjust the opticalaxis of an optical beam by using a surface elastic wave resulting fromentering the optical beam to an optical waveguide substrate 190 formedof LiNO₃, ZnO or the like.

[0172] Referring to FIG. 4B, if the frequency of a surface elastic waveexcited by a tooth-shaped electrode (transducer) 191 is adjusted, it ispossible to change the deflection angle of an optical beam. Thediffraction gratings 182 and 183 are provided so that the optical beamcan travel in/from the optical waveguide substrate 190.

[0173] Prior to the further description, an image forming process by theimage forming part (of the tandem type color image forming apparatus) isbriefly described with reference to FIG. 2.

[0174] In FIG. 2, optical beams emitted by the illuminant units 127 and128 are transmitted through the cylindrical lenses 129 and 130. Then,the optical beams are deflected by the movable mirror modules 131 and132 in such two directions that the optical beams can enter oppositesurfaces of the polygon mirror 126 to each other as illustrated in FIG.2. Here, the movable modules 131 and 132 have the same configuration asthe movable mirror associated with FIG. 3.

[0175] As discussed in association with FIG. 1, each of the illuminantunits 127 and 128 emits two optical beams, and the two optical beams areseparated into the upper beam and the lower beam with respect to thesub-scanning direction. After the illuminant units 127 and 128 emit twopairs of upper and lower optical beams, the upper and lower opticalbeams enter the cylindrical lenses 129 and 130 at positionssymmetrically away from the center axes of the cylindrical lenses 129and 130 with respect to the sub-scanning direction. For each pair, theupper and the lower optical beams cross each other with respect to thesub-scanning direction in the vicinity of deflection reflection surfacesof the polygon mirror 126 and are deflected in the same direction by thepolygon mirror 126. Then, the deflected optical beams, which are emittedby the illuminant units 127 and 128, pass through the fθ lenses 133 and134, respectively. One of the two optical beams from the illuminant unit127 travels to the photoconductor drum 121 via the folding mirrors 135and 156, and the other optical beam travels to the photoconductor drum122 via the folding mirrors 136 and 137. The toroidal lenses 138 and 139are provided to the two optical beams separately. For the photoconductordrums 123 and 124, the optical beams from the illuminant unit 128 aresimilarly guided thereto via two folding mirrors and different toroidallenses. For simplicity, the folding mirrors, and the toroidal lenses forthe photoconductor drums 123 and 124 are not designated in FIG. 2 byreference numerals.

[0176] As mentioned previously, each of the illuminant units 127 and 128comprises a plurality of semiconductor lasers, coupling lens, a holderfor retaining a synthesizing prism, and a semiconductor laser drivecircuit whose print substrate is mounted on the back surface thereof.The illuminant units 127 and 128 are configured to be rotated withrespect to the cylindrical part from which optical beams are emitted.Through adjustment of the cylindrical part, it is possible to fine-tunea writing phase of each of the upper and the lower optical beams.

[0177] The individual photoconductor drums 121, 122, 123 and 124 aredirectly connected to motors 141, 142, 143 and 144, respectively. Themotors 141, 142, 143 and 144 are rotated clockwise at the same frequencyas illustrated in FIG. 5.

[0178] The transfer belt 125 is rotated counterclockwise by a drivingroller 145 connected to a motor 146 and is tensioned by driven rollers147 and 148.

[0179] In this embodiment, each of the photoconductor drums 121, 122,123 and 124 is in contact with the transfer belt 125 at a transferringposition. The transferring positions are arranged at an interval of anintegral multiple of the circumferential length of the driving roller145. In this configuration, even if the transfer belt 125 is shifted atperiodically varying speeds due to eccentricity of the driving roller145, it is possible to make the phase of the periodical speed variationuniform for the transferring positions. A detector 149 for reading areference position of each image is provided near the driving motor 145on the transfer belt 125.

[0180] The detector 149 comprises a CCD area sensor 151 and an objectivelens 150. Three detectors 149 are disposed near the center and the bothends of the transfer belt 125 along the main scanning direction. Thedetectors 149 enlarge and read a predefined detection pattern formed ofa reference color toner image (black toner image) and then detect resistpositions with respect to the main scanning direction and thesub-scanning direction. At the same time, by comparing the black tonerimage with the other color (cyan, magenta and yellow) toner images, thedetectors 149 detect “differences between resist positions”, “the slopeof a scanning line” and “the curvature of a scanning line”. Thedifferences between resist positions are derived based on the detectedresist positions with respect to the main scanning direction and thesub-scanning direction. The slope of a scanning line is derived based ona detected difference between the both ends. The curvature of a scanningline is derived based on a detected difference between the middle pointof the both ends and the center.

[0181] In order to detect a pitch variation, (not illustrated)caterpillar-shaped patterns are formed in the sub-scanning direction ata constant period, and intervals of the caterpillar-shaped patterns(time lag) are read as “streak misalignment” at a predefined samplingtime. The period and the amplitude of the shift speed variation of thetransfer belt 125 and phase differences between the speed variation andthe detected resist positions are detected through a frequency analysisof the read intervals.

[0182]FIG. 5 shows an exemplary relation between the writing position WRonto a photoconductor drum DR and the transferring position TR. In FIG.5, the rotational center O of the photoconductor drum DR is misalignedto the center axis of the photoconductor drum DR. The angle between thewriting position WR and the transferring position TR is set as α.

[0183] In this configuration, if rotation of the photoconductor drum DRhas no eccentricity and the rotational speed of the photoconductor drumDR is constant, it takes a constant time t for the photoconductor drumDR to be rotated from the writing position WR to the transferringposition TR.

[0184] However, the time t of the photoconductor drum DR may differ fromanother photoconductor drum due to “unevenness of the drum diameter” ofthe photoconductor drums DR and misalignment of the writing position WR.Also, if the photoconductor drum DR has the eccentric rotational centerO due to the fabrication inaccuracy, the time t would periodicallychange corresponding to the configuration of the photoconductor drum DR.

[0185] According to this embodiment, if variations of the time t arecaused by the drum diameter unevenness and the writing positionmisalignment as mentioned above, it is possible to correct thevariations by selecting an optimal illuminant to write the head lineamong a plurality of illuminants constituting a semiconductor array ofthe illuminant unit.

[0186]FIG. 6 shows a case where the semiconductor laser array of theilluminant unit has five illuminants. Five optical spots LD-1 throughLD-5 are formed of optical beams from the five illuminants at thewriting position WR on the photoconductor drum DR. When such asemiconductor laser array is used to perform a multi-beam type scanningprocess, five scanning lines to simultaneously scan a photoconductordrum are generated from the optical spots LD-1 through LD-5.

[0187] In FIG. 6, the notation N represents a surface of a polygonmirror for reflecting and deflecting the five optical beamssimultaneously, and the notation N+1 represents the next surface of thepolygon mirror. As shown in FIG. 6; whenever the surface for reflectingand deflecting optical beams is switched through rotation of the polygonmirror, the five scanning lines to simultaneously scan a photoconductordrum are formed on the drum surface.

[0188] When image data are written with the writing position WR on thephotoconductor drum DR, one optical spot for writing the head line isselected among the optical spots LD-1, LD-2, LD-3, LD-4 and LD-5 thatare deflected on a same surface of the polygon mirror. The selectedoptical spot has a minimum difference with the resist position(right-hand side in FIG. 6) of a reference color detected by thedetector 149. In this case, only two scanning lines generated from theoptical spots LD-4 and LD-5 are used for the surface N of the polygonmirror, that is, at the start time of writing. Then, the five scanninglines are used for the next surface N+1 of the polygon mirror.

[0189]FIG. 7 is a block diagram of a circuit for selecting an illuminantto write the head line. In FIG. 7, a plurality of illuminants of asemiconductor laser array are referred to as the same notations as theabove-mentioned optical spots LD-1 through LD-5. As shown in FIG. 7,when a first multiplexer MP1 receives image data, the first multiplexerMP1 distributes each five lines of the image data to five buffermemories M1 through M5, and then the distributed five lines aretemporarily stored therein.

[0190] A second multiplexer MP2 selects a semiconductor laser to writethe head line based on reference position data. Then, the secondmultiplexer MP2 reads the stored image data from the buffer memories M1through M5 synchronously with a synchronizing signal of each surface ofthe polygon mirror and drives the illuminants LD-1 through LD-5 of thesemiconductor laser array via a writing control part WCT. Here, if aportion of the image data has not been written in deflection on asurface of the polygon mirror, the portion is stored until deflection onthe next surface of the polygon mirror.

[0191] When the optical spot to write the head line is selected so thatthe difference with the resist position of the reference color can beminimized, it is possible to properly correct differences of the time tamong the individual photoconductor drums due to the drum diameterunevenness and the misalignment of the writing positions.

[0192] In this case, by using the synchronizing signals as triggers, thewriting control part resets a main scanning directional timing forwriting an image for each optical scanning part of the image formingapparatus based on detection results of resist positions such that thecenter of the reference color coincides with the center of the eachimage region. In order to make the image widths (total widthmagnification) uniform, an image frequency for modulating asemiconductor laser is multiplied in inversely proportion to variationsof the image widths. Then, it is possible to properly superpose theindividual image regions by selecting the closest setting value amongvalues determined by individual division rates.

[0193] The resist positions are periodically set with respect to themain scanning direction and the sub-scanning direction during a start-upperiod before an image forming job or a waiting period between jobs suchthat the resist positions are suitable to an environment of the imageforming apparatus.

[0194] A description will now be given of correction of periodicalvariations of the time t due to eccentricity of the rotational center ofa photoconductor drum or a transfer belt.

[0195] In FIG. 5, an eccentric angle γ indicates an angle between thewriting position WR and the rotational center O of the photoconductordrum. In order to correct the periodic variations of the time t due tothe eccentricity of the rotational center of the photoconductor drum orthe transfer belt, the above-mentioned movable mirror is slightlyvibrated at the period of one rotation of the photoconductor drum DRcorresponding to the eccentric angle γ such that the amplitude and phaseof the movable mirror coincide with those of the periodical variations.As a result, it is possible to periodically change the writing positionso that the time t can be constant.

[0196] If the center axis of the photoconductor drum is misaligned tothe rotation axis thereof, the distance r between the rotation axis andthe circumferential surface of the photoconductor drum is varied in therange of r₀±Δr, where r₀ is the drum diameter without eccentricity andΔr is an amount of the eccentricity. In this case, when thephotoconductor drum is rotated at a constant speed, the photoconductordrum has different surface speeds corresponding to positions on thecircumferential surface thereof. If the surface speed is higher at aposition on the circumferential surface, the writing position is shiftedaway from the transferring member, that is, in the rotational upstreamdirection thereof. In contrast, if the surface speed is lower at aposition on the circumferential surface, the writing position is shiftedto the transferring member. This adjustment makes the rotation time t ofthe photoconductor drum from the writing position to the transferringposition uniform. In other words, even if the writing position is notshifted, it is possible to make the time t, which is varied over time,uniform by slightly changing the time t.

[0197] The surface speed around the writing position is periodicallyvaried at the period of one rotation of the photoconductor drum. Thus,if the movable mirror in FIG. 3 is used to deflect an optical beam inthe sub-scanning direction synchronously with the period and to shiftthe writing position such that the varying surface speed matches thephase of the period, it is possible to make the time t uniform. At thistime, an optical beam is sufficiently slowly varied on the movablemirror with respect to the sub-scanning direction relative to the periodof the optical scanning. In this case, although the optical beam isdeflected with respect to the sub-scanning direction, only extremelyslight curvature and slope are caused in the scanning line correspondingto the deflected optical beam. As a result, it is possible to ignore thecurvature and slope of the scanning line in practice.

[0198]FIG. 8A shows an exemplary variation of a recording pitch on animage with respect to the sub-scanning direction. This variation isgenerated as a composition of large undulations as illustrated in FIG.8B and small undulations as illustrated in FIG. 8C. The largeundulations are generated during one rotational period of aphotoconductor drum, and the small undulations are generated during onerotational period of a driving roller for driving a transfer belt.

[0199] The above description handles the case where the movable mirrorin FIG. 3 is used to correct, in particular, the large undulationsthrough the periodical deflection of an optical beam with respect to thesub-scanning direction. As shown in FIG. 8A, however, if the recordingpitch is varied in accordance with the composite of the two oscillationmodes, the above-mentioned movable mirror module in FIG. 4A ispreferably used. The movable frame 184 is vibrated synchronously withthe large undulations, and the movable mirror 181 in FIG. 4A is vibratedsynchronously with the small undulations. As a result, it is possible toaccurately shift the writing position with respect to the sub-scanningdirection in response to the composite undulations as shown in FIG. 8A.Therefore, it is possible to realize the constant time t even if thevariation of the recording pitch in FIG. 8A exists.

[0200] However, as mentioned above, even if a movable mirror is vibratedat the period corresponding to only the larger undulation of the twodifferent oscillation modes, it is possible to effectively reduceunevenness of the recording pitches due to the periodical variations ofthe time t by making the amplitudes and the phases uniform so as tooffset the pitch variations at the transferring position.

[0201] Since the above-mentioned pitch variations are characteristicsinherent in an image forming apparatus with a drive transmission system,it is possible to effectively reduce the unevenness of the pitches bymaking the amplitudes and the phases uniform at the initial arrangementso as to offset the pitch variations at each transferring position.

[0202] In FIGS. 8A through 8C, the speed is varied at the constantperiod. However, even if the period of the speed variations sporadicallychanges during an image forming process due to load variations andothers, it is possible to correct the pitch unevenness like the abovefashion by inclining the movable mirror at an occurrence timing of thesporadic period change.

[0203]FIG. 9 is a block diagram of a drive circuit for driving a movablemirror. A voltage supplied to individual electrodes 204, whichcorrespond to electrodes of the movable mirror module illustrated inFIG. 3, is generated as follows. Based on a reference clock, a pulsewidth generating part 201 generates a sequence of pulses correspondingto an oscillation period of the movable mirror.

[0204] In a PLL circuit 202, the phase of the pulse sequence is madesynchronous with a reference position detecting signal of the rotationaldirection of the photoconductor drum. The reference position detectingsignal is generated based on an encoder of a drive motor for thephotoconductor drum or a detected resist mark on the transfer belt. Again adjusting part 203 sets the oscillation amplitude of the movablemirror corresponding to an amount of speed variations (an amount ofspeed variations at the writing position on the circumferential surfaceof the photoconductor drum), and then the resulting voltage is appliedto the electrodes 204.

[0205] The pulse width generating part 201 is capable of generating apulse sequence whose frequency is varied over time and a sporadic pulse.Thus, the pulse width generating part 201 can change the oscillationperiod of the movable mirror corresponding to correction data.

[0206] A supplemental description will now be given, with reference toFIG. 2, of the image forming part of the optical scanner according tothe present invention.

[0207] The fθ lenses 133 and 134 are hybrid lenses in a sense that thefθ lenses 133 and 134 are formed by attaching aspheric components toglass-grinded spherical lenses by resin molding. On the other hand, thetoroidal lenses 148 and 149 are formed by injection molding.

[0208] Although toroidal lenses are provided corresponding to each ofthe photoconductor drums 121 through 124, all the toroidal lenses havethe same configuration as the toroidal lens 138. Thus, the followingdescription is focused on the toroidal lens 138. The toroidal lens 138comprises a lens part 152, a box-shaped rib part 154 for surrounding thelens part 152, and a flange part 153 that projects from both ends of thebox-shaped rib part 154 in the main scanning direction. A gate part 155,which is formed by injecting resin, is provided to one end of the flangepart 153.

[0209] Since the toroidal lens 138 has a long body, the lens part 152can have its own peculiarity that is generated at formation time. Forexample, the lens part 152 may be uniformly curved due to differences ofcooling time in local areas of the lens part 152 after injectionmolding. For this reason, the four toroidal lenses 148, 149 and others,which are provided corresponding to the photoconductor drums 121 through124, are disposed such that the gate parts of the toroidal lenses havethe same orientation and the curvatures thereof are similarly oriented.In this configuration, it is possible to make curvature directions ofscanning lines, which are caused by the curvatures of the toroidallenses, uniform to the individual photoconductor drums.

[0210] The flange part 153 has a flat plate body. In this configuration,the flange part 153 has a sub-scanning directional section modulus lowerthan that of the lens part 152 reinforced by the rib part 154 so as toabsorb torsional stress. In this fashion, it is possible to prevent thelens part 152 from twisting largely by using the release stress againstthe torsional stress.

[0211] A description will now be given, with reference to FIG. 10, of acorrecting method for correcting the slope of a scanning line.

[0212]FIG. 10 is a diagram for explaining the correcting method forcorrecting the slope of a scanning line wherein the toroidal lens 138 inFIG. 2 is referred to as the different numeral 211.

[0213] For instance, when a scanning line for optically scanning thephotoconductor drum 121 is inclined, this inclination is corrected byrotating the toroidal lens 211 in the optical axis direction in a statewhere the toroidal lens 211 is in contact with the surface orthogonal tothe optical axis.

[0214] The individual toroidal lenses 211 are disposed to face thephotoconductor drums 121 through 124, as illustrated in FIG. 10, and arearranged on the bottom surface 210 (a base member shared by the toroidallenses 211) of a housing such that the toroidal lenses 211 are uniformlyaligned with respect to the optical axis direction and the sub-scanningdirection. The toroidal lenses 211 are positioned with respect to themain scanning direction by catching projections 221, which projecttoward the photoconductor drums, at the center of the toroidal lenses211 with concave parts 223 on the bottom surface 210 of the housing. Thetoroidal lenses 211 are connected to the housing via the projections 221and concave parts 223 on the bottom surface 210 of the housing so as tobe fixed with respect to the main scanning direction.

[0215] The toroidal lenses 211 are depressed to eccentric cams 214 and215, which are in contact with one side surface of both ends of theflange parts 212, by first blade springs 225 so as to be positioned withrespect to the sub-scanning direction. Also, the under surfaces of thetoroidal lenses 211 are depressed to a housing reference surface 222 bysecond blade springs 224 so as to be positioned with respect to theoptical axis direction.

[0216] The eccentric cam 215 is screwed at installation time to serve asa reference eccentric cam. On the other hand, the other eccentric cam214 is used as an adjusting eccentric cam. The eccentric cam 214 isrotated via a pair of oblique tooth gears 216 by a pulse motor 217. Inthis configuration, even if the under surfaces of the toroidal lenses211 are in contact with the housing reference surfaces 222, the toroidallenses 211 can be initially set to be aligned at scanning positions ofoptical beams with respect to the sub-scanning direction by using thereference eccentric cam 215. As a result, it is possible to adjust theslopes of the scanning lines by using the adjusting eccentric cam 214.Additionally, it is possible to detect and correct a sub-scanningdirectional slope due to misalignment between the optical scanning partsor axial inclination of a photoconductor drum at any time based on arecorded image on the transfer belt.

[0217] In this embodiment, a black image is set as the reference colorimage, and the other color images are adjusted relative to the referencecolor image. Thus, the optical scanner for writing the black image isconfigured such that eccentric cams in contact with the toroidal lens211 for the photoconductor drum 121 are fixed with screws.

[0218] Alternatively, the reference eccentric cam 215 may be configuredto be rotated by a pulse motor like the adjusting eccentric cam 214.Thereby, it is also possible to correct the above-mentioned resistposition with respect to the sub-scanning direction.

[0219] A description will now be given, with reference to FIGS. 11A and11B, of a correction method for correcting curvature of a scanning line.FIGS. 11A and 11B are diagrams for explaining a correction method forcorrecting curvature of a scanning line.

[0220] As discussed associated with the image forming apparatus in FIG.2, optical beams enter a same surface of a polygon mirror and aredeflected so as to optically scan different photoconductor drums. Sincethe deflected optical beams cross around the surface of the polygonmirror with respect to the sub-scanning direction, the optical beams areinclined to a plane orthogonal to the rotational axis of the polygonmirror. Thus, the scanning line is not linear and is curved more greatlyat an end in the sub-scanning direction than at the center.

[0221] The scanning line may be curved due to an axial differencebetween the fθ lens and the toroidal lens, which serve as the imageforming parts. In this embodiment, the curved scanning line on aphotoconductor drum is made linear by curving the toroidal lens 211inversely to the curvature of the curved scanning line.

[0222] This correction process is described by using the toroidal lens211 as an example with reference to FIGS. 11A and 11B and FIGS. 12A and12B. FIGS. 12A and 12B show the supported toroidal lens.

[0223] As shown in FIGS. 12A and 12B, the flange parts 212, which areprovided at both ends of the toroidal lens 211, is in contact with thehousing reference surface 222 so as to support the toroidal lens 211. Asseen from a cross-sectional view of the toroidal lens 211 in FIG. 12B,the toroidal lens 211 is supported without any constraint to the lenspart with respect to the sub-scanning direction.

[0224] As shown in FIGS. 11A and 11B, an arch-shaped shape-memory metalplate 218 is connected at both ends thereof to one of the rib part 212of the toroidal lens 211.

[0225] As shown in FIG. 10, a thin-film resistor 219 is attached to aportion of the outer surface of the shape-memory metal plate 218. Bycontrolling electric power supplied to the thin-film resistor 219, it ispossible to change the temperature of the shape-memory metal plate 218.The curvature of the shape-memory metal plate 218 can be arbitrarilychanged depending on the temperature, resulting in elastic stress. Ifthe elastic stress, which is generated by the curvature variations,affects the toroidal lens 211 in the main scanning direction, it ispossible to adjust the curvature of the toroidal lens 211.

[0226] In other words, when the shape-memory metal plate 218 expands(contracts), the toroidal lens 211 is curved to be convex (concave)toward the shape-memory metal plate 218. If the shape-memory metal plate218 is made in advance to memorize such a curvature that the elasticstress to either of the two directions is generated at a lowertemperature, it is possible to correct the curvature of a curvedscanning line, which is a focal line with respect to the sub-scanningdirection, on a photoconductor drum through curvature offset even atnormal temperature.

[0227] Here, the shape-memory metal plate 218 and the thin-film resistor219 serve as a stress generating part.

[0228] If a scanning line is always curved in the same direction bytemperature variations, it is possible to set the curvature direction ofthe toroidal lens 211 in advance. As a result, it is possible toeffectively reduce the curvature of the scanning line without thethin-film resistor 219.

[0229] Alternatively, if shape-memory metal plates 218 are provided toboth outer surfaces of the rib part 212 with respect to the sub-scanningdirection, it is possible to offset the curvature of the scanning lineby properly taking a balance between the two shape-memory metal plates218.

[0230] In this configuration, the curvature may be adjusted by attachinga piezoelectric element to an arch-shaped plate. It is noted that theplate does not have to be a shape-memory metal plate in this case. Aslong as compression stress with respect to the main scanning directionis imposed between at least arbitrary two points of the toroidal lens211, it is possible to obtain the similar correction effect on thecurvature of a scanning line.

[0231]FIG. 13 shows an exemplary image forming apparatus in which theabove-mentioned optical scanner is installed wherein the photoconductordrums 121 through 124 in FIG. 2 are collectively referred to as thenumeral 241.

[0232] The most left photoconductor drum 241 (on which a black image iswritten) in FIG. 13 is described as an example. An electrifying charger242, a developing roller 243, a toner cartridge 244 and a cleaning case245 are disposed in the vicinity of the photoconductor drum 241. Theelectrifying charger 242 electrifies the photoconductor drum 241 at ahigh voltage. The developing roller 243 makes an electrostatic latentimage written by an optical scanner 240 visible by attaching chargedtoners to the electrostatic latent image. The toner cartridge 244supplies toners to the developing roller 243. The cleaning case 245cleans up and collects remaining toners on the photoconductor drum 241therein after a transferring process.

[0233] In the above-mentioned fashion, an image is written on thephotoconductor drum 241 by a multi-beam scanning operation using aplurality of scanning lines simultaneously per one surface of a polygonmirror. In the above-mentioned embodiment, five scanning lines are usedsimultaneously.

[0234] The four photoconductor drums 241 and the peripheral devices arearranged parallel to the shifting direction of a transfer belt 246. Eachcolor toner images of yellow, magenta, cyan and black are sequentiallytransferred and superposed onto the transfer belt 246 at an appropriatetiming so as to form a full color image.

[0235] A record paper (a recording medium) onto which a color image isto be transferred and fused is delivered from a paper tray 247 by apaper roller 248. A pair of resist rollers 249 delivers the record paperwhen an image starts to be recorded. The full color image on thetransfer belt 206 is transferred onto the delivered record paper. Thetransferred full color image on the record paper is fused by fusingrollers 250 and then is brought out in an output tray 251 by outputrollers 252.

[0236] According to this embodiment, a difference of resist positions isdetected from an image recorded on the transfer belt 125 as illustratedin FIG. 2. Based on the detection result, an illuminant to emit anoptical beam for writing the head line is selected. However, the imageforming apparatus according to the present invention is not limited tosuch selection. For instance, the illuminant for writing the head linemay be selected in the interior of the optical scanner through positiondetection of optical beams with respect to the sub-scanning direction.

[0237] A sensor part 230 in FIG. 10 detects the position of an opticalbeam with respect to the sub-scanning direction. The similarlyconfigured sensor part 230 is provided for each of the photoconductordrums 121 through 124.

[0238]FIG. 14 is a diagram for explaining the sensor part 230 fordetecting the position of an optical beam with respect to thesub-scanning direction.

[0239] The sensor part 230 has two PIN photodiodes 231 and 232 each ofwhich has a linear acceptance surface. As shown in FIG. 14, the PINphotodiode 231 is disposed such that the longitudinal axis of theacceptance surface thereof is parallel to the sub-scanning direction,and the PIN photodiode 232 is disposed such that the longitudinal axisof the acceptance surface thereof is inclined to the sub-scanningdirection. For each of the photoconductor drum 121 through 124, thesensor part 230 is provided and is mounted on a position of a commonsurface 233 out of a writing area of an optical path from the toroidallens 211 to the photoconductor drum 121 as illustrated in FIG. 10. Thecommon substrate 233 is fixed with screws 236 on the housing bottomsurface 210 at the reference side of the above-mentioned slopeadjustment of the toroidal lens 211 and others.

[0240] As shown in FIG. 14, each of the scanning optical spots LD-1through LD-5 is projected to have a predetermined pitch with respect tothe sub-scanning direction (the vertical direction in FIG. 14) with theadjacent optical spots. Thus, the optical spots LD-1 through LD-5 havedifferent detection time t1 through t5, respectively, between the PINphotodiodes 231 and 232, as illustrated in FIG. 14. If the optical spotsLD-1 through LD-5 are misaligned with respect to the sub-scanningdirection, the detection time differences t1 through t5 are uniformlyvaried.

[0241] Accordingly, if an illuminant for forming an optical spot whosetime difference is the closest to a predetermined time difference t₀ isselected, it is possible to achieve a minimum amount of misalignment ofthe head line and maintain an appropriate pitch between writing parts.Additionally, although the sensor part 230 is disposed at one end of thebottom surface 210 of the housing in the above-mentioned embodiment asillustrated in FIG. 10, two sensor parts 210 may be disposed at bothends of the bottom surface 210 with respect to the main scanningdirection so as to detect not only misalignment with respect to thesub-scanning direction but also slopes of scanning lines.

[0242] The housing for retaining optical elements of the optical scanneris often formed of a material whose thermal expansion coefficient isrelatively large, for example, almi-diecast. For this reason, when thetemperature of the interior of the housing rises, there is a risk thatthe housing bottom surface 210 may be curved, as illustrated in FIG. 15,in the lower direction from fastening parts 218 at both ends thereof.

[0243] If the housing bottom surface 200 is curved due to a temperatureincrease, there arises a risk that braces 226 for supporting the foldingmirrors 112 and 114 (135 and 156 in FIG. 2, respectively), which aremounted on the housing bottom surface, may be also inclined. As aresult, the reflection angle of the folding mirrors 112 and 114 may haveinappropriate reflection angles. In this case, however, the foldingmirrors 112 and 114 face each other. Thus, even if the reflection anglesare changed, the change of reflection angles can be offset because thefolding mirrors 112 and 114 are mutually displaced in the inversedirection. As a result, it is possible to effectively reduce variationsof optical spots due to the curvature of the housing bottom surface 210.

[0244] The misalignment of the reaching positions of the optical beamsis detected as a position difference of a scanning line with respect tothe sub-scanning direction. Thus, it is not necessary to use the twofolding mirrors 112 and 114 in the above fashion. One folding mirror mayguide an optical beam deflected by the polygon mirror to the toroidallens.

[0245] Additionally, although the convex surface of the toroidal lensfaces a photoconductor drum in this embodiment, the concave surface ofthe toroidal lens may face the photoconductor drum.

[0246] A description will now be given, with reference to FIG. 16, of animage forming apparatus according to another embodiment of the presentinvention.

[0247] Referring to FIG. 16, the image forming apparatus is a tandemtype color printer. Four photoconductor drums 301Y, 301M, 301C and 301B,which serve as image carriers, are arranged along the shift direction ofa transfer belt 310, which serves as a transferred member. The imageforming apparatus transfers different color toner images formed on thephotoconductor drums 301Y, 301M, 301C and 301B sequentially onto thetransfer belt 310 so as to form a full color image.

[0248] For example, the photoconductor drum 301Y has an electrifyingcharger 302Y, a developing roller 303Y, a toner cartridge 304Y and acleaning case 305Y in the vicinity thereof. The electrifying charger302Y electrifies the photoconductor drum 301Y at a high voltage. Inorder to make the resulting image visible, the developing roller 303Yattaches yellow toners to an electrostatic latent image recorded by anoptical scanner 300. The toner cartridge 304Y supplies yellow toners tothe developing roller 303Y. The cleaning case 305Y removes and collectsremaining toners on the photoconductor drum 301Y. An image formingstation for forming a yellow image has the above-mentionedconfiguration.

[0249] In this configuration, a plurality of lines of a source image(five lines in this embodiment) are simultaneously recorded on aphotoconductor drum 301 for each surface of a polygon mirror 306 asmentioned later. As shown in FIG. 16, the above-mentioned image formingstations are arranged along the shift direction of the transfer belt 310as the transferred member. Individual color toner images, that is, ayellow (Y) toner image, a magenta (M) toner image, a cyan (C) tonerimage and a black (B) toner image, are sequentially transferred andsuperposed onto the transfer belt 310 at an appropriate timing so as toform a full color image. Although the yellow image forming station hasbeen intensively described, it is understood that each of the othercolor image forming stations fundamentally has the same configuration asthe yellow image forming station except that different color toners areused. Regarding the other color image forming stations, thus, if thecolor indicative suffix Y is replaced with the other color indicativesuffixes M, C and B, the other color image forming stations can bedescribed in the similar fashion to the above-mentioned yellow imageforming station and, therefore, a description thereof is omitted herein.

[0250] A record paper, which serves as a recording medium, is deliveredfrom a paper trey 314 by a feeding roller 315. Then, a pair of resistrollers 316 sends the record paper in the sub-scanning directionsynchronously with a record start timing. A color image is transferredfrom the transfer belt 310 onto the record paper. The transferred recordpaper is fused by a fusing roller 317 and is supplied to an output trey319 by a pair of output rollers 318.

[0251] A description will now be given, with reference to FIG. 17, ofcooperation between the individual photoconductor drums 301Y, 301M, 301Cand 301B and the optical scanner 300. In this embodiment, individualoptical scanners for scanning each of the four photoconductor drums301Y, 301M, 301C and 301B are integrally configured as the singleoptical scanner 300. A polygon mirror 306 serves as a single deflectingpart and is operative for all optical beams toward the photoconductordrums 301Y, 301M, 301C and 301B.

[0252] Two illuminant units 330 and 331 serve as illuminant parts. Eachof the illuminant units 330 and 331 accommodates a pair of semiconductorlasers for emitting optical beams to be deflected on same surfaces ofthe polygon mirror 306. Each of the illuminant units 330 and 331 emitsoptical beams to the polygon mirror 306 such that the optical beams fromthe illuminant unit 330 enter the polygon mirror 306 opposite to thosefrom the illuminant unit 331. After being deflected by the polygonmirror 306, the optical beams from the illuminant units 330 and 331travel in different directions as illustrated in FIG. 17.

[0253] The optical beams from the illuminant units 330 and 331 arrive atthe polygon mirror 306 through movable mirror modules 334 and 335, whichserve as optical axis adjusting parts, and are deflected in thedirections opposite to each other by the polygon mirror 306. Asmentioned above, each of the illuminant units 330 and 331 emits twooptical beams from the two semiconductor lasers therein such that thetwo optical beams travel separately with respect to the verticaldirection. These optical beams, that is, the upper optical beams and thelower optical beams, from the illuminant units 330 and 331 enter samecylinder lenses 332 and 333, respectively, symmetrically away from thecenter axes of the cylinder lenses 332 and 333.

[0254] After being deflected by the polygon mirror 306, the deflectedoptical beams, which travel in the opposite directions each other, passthrough similarly configured fθ lenses 336 and 337. As shown in FIG. 17,one of the two optical beams from the illuminant unit 330 is guided tothe photoconductor drum 301Y via folding mirrors 338 and 339, and theother optical beam is guided to the photoconductor drum 601M via foldingmirrors 340 and 341. Two toroidal lenses 342 and 343 are separatelyprepared for each of the two optical beams from the illuminant unit 330.On the other hand, the two optical beams from the illuminant unit 331are guided to the photoconductor drums 301C and 301B in the samefashion. An image forming part of the image forming apparatus accordingto this embodiment comprises the fθ lenses 336 and 337, the foldingmirrors 338, 339, 340 and 341, and the toroidal lenses 342 and 343.

[0255] A printed board is installed to the rear surface of each of theilluminant units 330 and 331. The printed board accommodates thesemiconductor lasers, a holder for holding coupling lenses and asynthesizing prism, and a drive circuit for driving the semiconductorlasers. The illuminant units 330 and 331 can be rotated on a cylinderpart from which optical beams are emitted to the exterior thereof, andthereby it is possible to fine-tune the positions of the upper and thelower optical beams with respect to the main scanning direction.

[0256] The fθ lenses 336 and 337 are hybrid lenses in a sense that eachof the fθ lenses 336 and 337 is formed by coupling a thin-film asphericcomponent to a glass-grinded cylindrical lens by resin forming. Each ofthe toroidal lenses 342 and 343 integrally comprises an injection-moldedlens part 345, a box-shaped rib part 348 to enclose the lens part 345,and a flange part 346. The flange part 346 projects from both ends ofthe rib part 348 with respect to main scanning direction. A gate part347 from which resin is injected at molding time is provided to one endof the flange part 346.

[0257] Since the toroidal lenses 342 and 343 have wide bodies asillustrated in FIG. 17, there is a risk that the bodies may be uniformlycurved depending on a condition of the molding. For example, when thetoroidal lenses 342 and 343 are cooled down after the injection process,the toroidal lenses 342 and 343 may be uniformly curved depending ondifferences of cooling time in local areas of the bodies. For thisreason, all the toroidal lenses of the image forming parts are uniformlyoriented to the gate parts 347 thereof. Furthermore, the flange part 346is shaped as a flat body so as to have a lower section modulus withrespect to the sub-scanning direction than the lens part 345 reinforcedby the rib part 348. Thereby, even if the toroidal lenses 342 and 343receive some twist stress, it is possible to absorb the twist stress atthe flange part 346.

[0258] The photoconductor drums 301Y, 301M, 301C and 301B are directlycoupled to the axis of each motor and are rotated at a same drivefrequency in the rotational direction as indicated by the curved arrowsin FIG. 17. The transfer belt 310 is tensioned under predeterminedtensile force by a driving roller 350 and two driven rollers 351 and352. The driving roller 350 is rotated in the direction opposite to thephotoconductor drums 301Y, 301M, 301C and 301B by a motor 353 connectedthereto.

[0259] In this embodiment, each of the photoconductor drums 301Y, 301M,301C and 301B is in contact with the transfer belt 310. The contactpoint is called as a transferring position. The transferring position isset such that an interval between adjacent transferring positionsbecomes an integer-multiple of the circumferential length of the drivingroller 350. As a result, even if the driving roller 350 has a periodicspeed variation due to eccentricity thereof and other factors, it ispossible to apply the same phase of the periodic speed variation to eachof the photoconductor drums 301Y, 301M, 301C and 301B.

[0260] Furthermore, as shown in FIG. 17, three detectors 360 aredisposed at the center and the both ends of the transfer belt 310. Thedetectors 360 serve as resist difference detecting parts to detectreference positions of individual color images on the transfer belt 310.

[0261] Each of the detectors 360 comprises a CCD area sensor 361 and anobjective lens 362. The detector 360 detects a cross-shaped detectionpattern whose lines are drawn parallel to the main scanning directionand the sub-scanning direction as illustrated in FIG. 17. The detectionpattern is formed of color toner images of the reference color (black)and the other colors (cyan, magenta and yellow). The three detectors 360are capable of detecting an amount of a resist difference with respectto the sub-scanning direction based on the sub-scanning directional lineof the cross-shaped detection pattern. Additionally, the detectors 360are capable of detecting the slope of a scanning line based on adifference between two cross points of the cross-shaped detectionpatterns at the both ends of the transfer belt 310. Furthermore, thedetectors 360 are capable of detecting the curvature of a scanning linebased on a difference between the cross point of the detection patternplaced at the center of the transfer belt 310 and the middle point ofthe two cross points of the detection patterns at the both ends of thetransfer belt 310.

[0262]FIG. 18 shows the structure of an optical scanning part, which isa portion of the optical scanner 300, for the photoconductor drum 301Yin detail. Referring to FIG. 18, the illuminant unit 330 (331) comprisessemiconductor lasers 371 and 372, coupling lenses 373 and 374 and asynthesizing prism 375. Each of the semiconductor lasers 371 and 372exposes a photoconductor drum different from each other. Thesynthesizing prism 375 comprises a parallelogram part and a trapeziumpart.

[0263] Two optical beams from the semiconductor lasers 371 and 372 areconverted into parallel luminous fluxes by the coupling lenses 373 and374. The parallel luminous flexes enter the same surface of thesynthesizing prism 375. The optical beam from the semiconductor laser371 directly passes through the synthesizing prism 375, and, on theother hand, the optical from the semiconductor laser 372 is reflected ona pair of parallel reflection surfaces of the parallelogram part. Thereflected optical beam travels adjacently to the optical beam from thesemiconductor laser 371 such that the two optical beams have apredetermined convergence angle with respect to the sub-scanningdirection.

[0264] After the two optical beams temporarily cross each other, theoptical beams enter a cylinder lens 332 from positions away from thecenter axis of the cylinder lens 332, resulting in parallel opticalbeams of an interval of about 3 mm with respect to the sub-scanningdirection. The optical beams enter the polygon mirror 306 via a movablemirror module 334. The movable mirror module 334 (335) serves as anoptical axis adjusting part for adjusting the optical axes of theoptical beams with respect to the sub-scanning direction throughrotation on a twisted beam thereof as the rotation axis. After beingdeflected by the polygon mirror 306, one of the two optical beams isfocused as an optical spot on the photoconductor drum 301Y via an fθlens 336 and a toroidal lens 342. The fθ lens 336 is operative to theoptical beam with respect to the main scanning direction. The toroidallens 342 is operative to correct an optical face tangle error caused bythe polygon mirror 306.

[0265] During formation of an image, a constant voltage is being appliedto the movable mirror module 334 (335) so as to maintain movable mirrors376 and 377 at predetermined slope angles.

[0266] Each of the semiconductor lasers 371 and 372 includes a pluralityof illuminants. The illuminants are monolithically arranged at a pitch dof 10-20 μm. The illuminants may be arranged as an array in thesub-scanning direction. For the pitch d of the illuminants, if theabove-mentioned image forming optical system is designed such that thesub-scanning lateral magnification β meets the formula;

β=P/d(P: pixel pitch),

[0267] it is possible to position optical spots at the pixel pitch Pcorresponding to a recording density.

[0268] In this embodiment, since two optical beams from the fθ lens 336are separated at a predetermined interval with respect to the verticaldirection, it is possible to guide the optical beam from thesemiconductor laser 371 to the photoconductor drum 301Y by using thefolding mirrors 338 and 339. In FIG. 18, the optical path of the opticalbeam from the semiconductor laser 372 are omitted. Although the movablemirror module 334 (335) controls optical beams for two colorphotoconductor drums, one movable mirror module may be provided for eachcolor.

[0269] For each optical scanning part of the optical scanner 300, asynchronization detecting sensor 378 and an end detecting sensor 379,which constitute an optical detecting part, are disposed at the scanningstart side and the scanning end side of an image recorded area,respectively. The synchronization detecting sensor 378 detects a writingstart timing with respect to the main scanning direction. Throughmeasurement of scanning time between the both sensors, a change of animage width (width magnification) is detected. Image frequency formodulating the semiconductor laser 371 is multiplied inversely to thedetected image width so as to make the image width constant. As shown inFIG. 18, mirrors 380 and 381 are provided to guide the optical beam tothe synchronization detecting sensor 378 and the end detecting sensor379, respectively.

[0270] As shown in FIG. 19, each of the synchronization detecting sensor378 and the end detecting sensor 379 comprises a photodiode 391orthogonal to the main scanning direction and a photodiode 392 that isnot parallel to the main scanning direction. Just when an optical beampasses through an edge of the photodiode 391 of the synchronizationdetecting sensor 378/the end detecting sensor 379, a synchronizationdetecting signal/an ending detecting signal is generated. In order tofind a sub-scanning directional position difference Δy of the opticalbeam, a difference Δt of passage time from the photodiode 391 to thephoto diode 392 is measured.

[0271] Here, the sub-scanning directional position difference Δy isrepresented by the following formula;

Δy=(V/tanγ)·Δt,

[0272] where γ is the slope angle of the photodiode 392, and V is ascanning speed of the optical beam. If Δt is constant, it can beconcluded that there is no position difference with respect to thesub-scanning direction. In this embodiment, Δy is computed as theaverage of values at the scanning start side and the scanning end side.

[0273] As shown in FIG. 20A and 20B, the synchronization detectingsensor 378 and the ending detecting sensor 379 (optical detectingsensors) are supported by a mounting surface 410. FIG. 20A shows anexemplary optical detecting sensor that is installed horizontally to themounting surface 410, and FIG. 20B shows an exemplary optical detectingsensor that is installed orthogonally to the mounting surface 410. Inthe following, the horizontally installed optical detecting sensor inFIG. 27A is focused.

[0274] The synchronization detecting sensor 378 (or the end detectingsensor 379) is screwed on a substrate 402 with an L-shaped resin holder403. A step part 412 of an image forming lens 405 is caught to a squarehole 404 with a pair of snaps 406 such that the synchronizationdetecting sensor 378 is oriented to the same direction as the opticalaxis of the image forming lens 405.

[0275] As shown in FIG. 20A, a reference hole 407 and a long hole 408are provided on the bottom surface of the holder 403 so as to bepositioned at backward and forward sides of the scanning direction, asindicated by the arrow, respectively, with respect to the center of thesynchronization detecting sensor 378. A pin 411 is formed to projectfrom the mounting surface 410 of an optical housing. The pin 411 isinserted into the long hole 408, and the holder 403 is screwed to themounting surface 410 via the reference hole 407. Here, the end detectingsensor 379 has a holder that is configured symmetrically to the holder403 for the synchronization detecting sensor 378.

[0276] The holder 403 is formed of a material that satisfies thefollowing formula;

s·x=S·H,

[0277] where s is a thermal expansion coefficient of the holder 403, xis a distance between the synchronization detecting sensor 378 and thereference hole 407, S is a thermal expansion coefficient of the opticalhousing, and H is a distance between the center of an image with respectto the main scanning direction and the synchronization detecting sensor113, which is a synchronization image height in this embodiment. Underthis selection, even if the optical housing, which is formed ofalmi-diecast in this embodiment, extends due to temperature variations,the synchronization detecting sensor 378 has a steady sensor position.If S<s, a position difference is not large and therefore it is possibleto detect variations of an image width with high accuracy.

[0278] In this embodiment, a hybrid lens is adopted as the fθ lens 336.The fθ lens 336 is mainly formed of a glass-grinded lens whose focaldistance slightly varies in response to temperature variations.Alternatively, if a resin molded lens is used as the fθ lens 336, thefocal distance varies, especially, due to refractive index variationsand thermal expansion.

[0279] A change Δf of the focal distance is represented by the followingformula;

Δf={(Δn−k)/(n−1)}·f·ΔT,

[0280] where Δn is a change of the refractive index of a resin materialdue to temperature, k is a thermal expansion coefficient, n is therefractive index of the resin material, f is a focal distance of allsystems, and ΔT is a change of temperature.

[0281] Therefore, a difference Δθ of a scanning angle detected at asynchronization detecting position is represented by the followingformula;

Δθ={1/(1+n0·ΔT)−1}·(H/f),

[0282] where n0 is a temperature coefficient of the focal distance andn0=(Δn−k)/(n−1), and H is a synchronization image height. The writingposition has a difference corresponding to f·Δθ. However, if thedistance x between the synchronization detecting sensor 378 and thereference hole 407 and the thermal expansion coefficient s of the holder403 are set to satisfy the following formula;

s·x=n0·H,

[0283] it is possible to cancel the writing position difference becauseof free expansion of the holder 403. Although the position differencedue to variations of the focal distance is smaller than theabove-mentioned position difference due to the optical housing, the fθlens 336 may be designed under consideration of combination of theseposition difference.

[0284] A description will now be given, with reference to FIG. 21, ofthe structure of the movable mirror modules 334 and 335. The movablemirror module 334, (335) has a plurality of vibration modes (two modesin this embodiment) on a support substrate 421 shaped of a sinteredmetal. A first movable mirror 376 and a second movable mirror 377 areprovided to the movable mirror module 334 (335) so as to be arrangedparallel to the sub-scanning direction. The movable mirror module 334(335) is sealed by a cap-shaped cover 425. An optical beam travelsthrough a glass window 424 on an aperture of the cover 425.

[0285] A movable mirror substrate is configured by joining two Sisubstrates 426 and 427 via an insulation film. The first Si substrate426 is formed of an Si substrate of 60 μm in thickness. As shown in FIG.21, the first movable mirror 376, the second movable mirror 377, a firsttwisted beam 428 for supporting the first movable mirror 376 and asecond twisted beam 429 for supporting the second movable mirror 377 areformed by etching on a fixing frame 430. The first twisted beam 428 andthe second twisted beam 429 are disposed parallel to the main scanningdirection.

[0286] A shown in FIG. 21, teeth are formed at both edge parts of themovable mirrors 376 and 377. Teeth are formed at the opposite side ofthe fixing frame 430 to the teeth of the movable mirrors 376 and 377 soas to engage the teeth of the fixing frame 430 with the teeth of themovable mirrors 376 and 377.

[0287] A metal film such as Au is deposited on the surfaces of themovable mirrors 376 and 377 and the teeth of the fixing frame 430. Theteeth of both edges of the movable mirrors 376 and 377 serve as a firstand a second movable electrodes, respectively. The teeth of the fixingframe 430 opposite to the first and the second movable electrodes serveas a first and a second fixed electrodes, respectively. The surfaces ofthe movable mirrors 376 and 377 are deposited with an oxide film or thelike so as to have a stress difference between the front and the backsurfaces of the substrate. Thereby, the twisted beams 428 and 429 aretwisted so that the movable mirrors 376 and 377 can be inclined on morethan predetermined angle at unloaded time. When a voltage is applied tothe first and the second fixed electrodes, electrostatic force isgenerated between the fixed electrodes and the opposite movableelectrodes. As a result, the movable mirrors 376 and 377 are separatelyrotated by small angles in a direction where the movable mirrors 376 and377 are horizontally orientated, and the slopes of the movable mirrors376 and 377 are determined to be balanced with the twisted beams 428 and429, respectively.

[0288] In this embodiment, it is possible to shift a scanning positionby about 5 μm per the slope angle of 1′ of the movable mirrors 376 and377.

[0289] As mentioned later, as the twisted beams 428 and 429 have asmaller width c and a greater length L, the swing angle θ of the movablemirrors 376 and 377 becomes greater. However, if a resonance frequencyis low, the twisted beams 428 and 429 resonate due to vibrationtransmission involved in rotation of a drive motor and others.Therefore, it is preferable that the twisted beams 428 and 429 be set tohave a sufficiently high resonance frequency (more than 200 Hz).

[0290] It is supposed that the movable mirrors 376 and 377 have 2 a inlength, 2 b in width and d in height and the twisted beams 428 and 429have L in length and c in width. Then, the spring constant K of thetwisted beams 428 and 429 is given by the following formula;

K(G/2L)·{cd(c ² +d ²)/12},

[0291] where G is a material constant.

[0292] On the other hand, electrostatic force F between the electrodesis given by the following formula;

F=εHV ²/2 δ,

[0293] where ε is a dielectric constant of air, H is a length of theelectrodes, V is an applied voltage, and δ is a distance between theelectrodes. A rotation torque T for driving the movable mirrors 376 and378 is given by the following formula;

T=F·2b.

[0294] Thus, the swing angle θ is represented by the following formulae;

θ=T/K=B·bH·V ² /ε·{cd(c ² +d ²)/L},

[0295] where B is a constant.

[0296] As a result, when the applied voltage V is controlled, it ispossible to set the swing angle θ as a predetermined value.

[0297] From the above formulae, it is concluded that as the electrodelength H is larger, the swing angle θ is greater. As a result, as theelectrodes are formed as teeth as in this embodiment, it is possible toobtain 2n torque for the number n of teeth. In order to enhanceresponse, furthermore, a moment of inertia is attempted to be reduced.In order to reduce the moment of inertia, it is necessary to decreasethe weight of the movable mirrors 376 and 377 by cutting off the backside of the movable mirrors 376 and 377. In order to minimize influencesof viscous resistance of air such as damping, the movable mirrors 376and 377 are sealed with the cover 425 so that the sealed interior canhave low pressure.

[0298] The second substrate 427 is formed of an Si substrate of 200 μmin thickness. The second substrate 427 supports a support part of thefirst Si substrate 426 through the center aperture thereof asillustrated in FIG. 21. The center aperture provides a swing area forthe movable mirrors 376 and 377. Lead terminals 432 are inserted intothe support substrate 421 via an insulation member. Pad parts 433, whichare connected to individual fixed electrodes on the substrate surface,are connected to terminal parts, which project toward the upper sides ofthe lead terminals 432, by wire bonding. Thereby, electric wiringbetween the interior and the exterior of the sealed package isimplemented.

[0299] The cover 425 is embedded in the stepping part formed around thecircumference of the support substrate 421. The glass window 424 iscoupled to the aperture from the inner side of the cover 425. Althoughthe movable mirrors 376 and 377 are driven by electrostatic forceaccording to the above-mentioned embodiment, the movable mirrors 376 and377 may be driven by a piezoelectric element and the like.

[0300]FIG. 22 shows a relation between the writing position and thetransferring position on the photoconductor drum 301.

[0301] In FIG. 22, the notation O represents the rotational center ofthe photoconductor drum 301. The angle between the writing position andthe transferring position is set as α. As a result, if thephotoconductor drum 301 is rotated at a predetermined angular velocity,the photoconductor drum 301 is rotated at a constant time t from thewriting position to the transferring position.

[0302] Based on the above-mentioned detection pattern, the individualcolor resist positions with respect to the sub-scanning direction aredetected. For each resist position, the writing timing is periodicallyadjusted per a pitch P of a scanning line for every other surface of thepolygon mirror 306 so as to align the resist positions with respect tothe sub-scanning direction. Then, the diameter D of the photoconductordrum 301 is represented by the following formula;

D·α/2=N·P+ΔP,

[0303] where N is a natural number, and ΔP is a writing start timingdifference due to a phase difference between synchronization detectingtimings. Also, transferring position interval B between each of thecolor photoconductor drums and the reference color photoconductor drumare given by the following formula;

B=M·P+ΔP,

[0304] where M is a natural number.

[0305] Thus, even if D, α and B are different from each other, it ispossible to make the rotational speed constant. As long as the writingposition is not changed, only the difference ΔP between the individualwriting start timings persists.

[0306] The difference ΔP is at most half an pitch, that is, ΔP≦P/2, andthe optical axis of an optical beam is shifted in the sub-scanningdirection by ΔP by using the movable mirrors 376 and 377 to make thedifference ΔP zero.

[0307] Meanwhile, a semiconductor laser array, on which five illuminantsare monolithically formed, is used in this embodiment to simultaneouslyscan five lines for each surface of the polygon mirror 306. Also, inthis case, the writing start timings are adjusted in the same fashion.

[0308] As shown in FIG. 23, a plurality of beam spots are arranged atthe pitch P on a photoconductor drum 301 corresponding to the recordingdensity with respect to the sub-scanning direction. An optical beam forwriting the head position is selected among a plurality of optical beamsLD-1, LD-2, LD-3, LD-4 and LD-5 all of which are reflected on a samesurface of the polygon mirror 306. In this selection, the optical beamthat has the smallest difference with a reference color resist position(right side) is selected, that is, the optical beam LD-4 is selected inFIG. 23. Therefore, only the writing start timing difference ΔP persistsin this case. In FIG. 23, the notations N and N+1 indicate the surfacenumber of the polygon mirror 306.

[0309]FIG. 24 is a block diagram of a control mechanism for correcting adifference of writing starting timings. Every five lines of image dataare distributed by a first multiplexer. The lines are temporarily storedin a buffer memory. Based on reference position data, a secondmultiplexer selects an optical beam for writing the head row andswitches the current control into the semiconductor laser emitting theselected optical beam.

[0310] The stored data are read from the buffer memory synchronouslywith a synchronizing signal for each surface of the polygon mirror 306.Data that have not been recorded on this surface of the polygon mirror306 are stored until the next surface of the polygon mirror 306, andthen the data are recorded at the next surface of the polygon mirror306.

[0311] At this time, a writing control part resets a writing starttiming of an image with respect to the main scanning direction for eachimage forming station by using a synchronization detecting signal as atrigger. The writing control part determines the writing start timingbased on detection of resist positions with respect to the main scanningdirection such that the center position of an image area for thereference color coincides with the center positions of image areas forthe other individual colors. As a result, it is possible to properlysuperpose the individual image areas.

[0312] The above-mentioned sub-scanning and the main scanning resistpositions are periodically set suitably to an environment of an imageforming apparatus during start-up time before printing or a waiting timebetween jobs.

[0313] In this embodiment, the above-mentioned detection method of aresist position difference uses an image recorded on the transfer belt310. However, the detection method of a resist position difference isnot limited to the above-mentioned fashion based on the image on thetransfer belt 310. For instance, the sensor as shown in FIG. 19 may beused to detect the sub-scanning position of an optical beam in theoptical scanner, and the optical beam that is the closest to apredetermined reference value of the time difference Δt may be selectedas the optical beam for writing the head row.

[0314] Since each of the optical beams LD-1, LD-2, LD-3, LD-4 and LD-5scans away each other at a predetermined pitch with respect to thesub-scanning direction, differences t1 through t5 of detection timebetween the PIN photodiodes 391 and 392 are different from each other.If the sub-scanning position is misaligned, these differences t1 throught5 increase or decrease uniformly. Therefore, by selecting the opticalbeam closest to a predetermined reference value of the time differenceΔt and inclining the movable mirrors 376 and 377 so as to coincide withthe reference value of Δt, it is possible to make the positiondifference of the head row approximately 0 and maintain constantintervals between the individual color scanning positions.

[0315] For each image forming station, the synchronization detectingsensor 378 and the end detecting sensor 379 are supported in the opticalhousing by the holder 403. In order to guide optical beams to thesynchronization detecting sensor 378 and the end detecting sensor 379,as shown in FIG. 18, the optical beams are folded at the both ends outof the writing area of the optical path between the toroidal lens 342and the photoconductor drum 301Y by the mirrors 380 and 381 mounted tothe optical housing.

[0316]FIG. 25 is a block diagram of a drive circuit for driving themovable mirrors 376 and 377. Referring to FIG. 25, a voltage applied toindividual electrodes is increased or decreased by the gain adjustingpart depending on the time difference Δt detected by the above-mentionedsensors 378 and 379. The slopes of the movable mirrors 376 and 377 arefeedback-controlled such that the time difference Δt coincides with thepredetermined reference value thereof.

[0317] Also, the gain adjusting part periodically resets the referencevalue of the time difference Δt based on a resist difference δ detectedthrough a detection pattern recorded on the transfer belt 310.

[0318] In this embodiment, since the sensors 378 and 379 are disposed inthe vicinity of the photoconductor drum 301, the resist difference δ isapproximately in proportion to the time difference Δt. Thus, the timedifference Δt is determined based on the following formula;

Δt=j·δ,

[0319] where j is a predetermined coefficient.

[0320] Therefore, it is also possible to prevent the resist differenceon the photoconductor drum 301. Here, the slopes of the movable mirrors376 and 377 are set when the power supply is switched ON or betweenprinting jobs. At least, when an image is recorded, a voltage is appliedto maintain the slopes that have been set in advance.

[0321] As mentioned above, the optical scanner according to thisembodiment periodically resets the time difference Δt based on thedetected resist difference on the transfer belt 310 and monitors for theresist positions with respect to the sub-scanning direction by using thesensors 378 and 379 in the optical housing.

[0322]FIG. 26A shows an exemplary variation of a recording pitch on animage with respect to the sub-scanning direction. This variation isgenerated, for example, from a composite of large undulations in FIG.26B and a small undulations in FIG. 26C. The large undulations in FIG.26B are caused by a speed variation of the photoconductor drum 301 perthe period of one rotation thereof, and the small undulations in FIG.26C are caused by a speed variation of the transfer belt 310 per theperiod of one rotation of the driving roller 350. Even if individualwriting timings coincide with each other, the resist positions areperiodically varied due to the speed variations.

[0323] In order to reduce the resist difference due to the speedvariations, the individual color transferring positions areconventionally set to have intervals (station intervals) equal to anintegral multiple of the circumferential lengths of both thephotoconductor drum 301 and the driving roller 350. As a result, it ispossible to make the circumferential speeds of the photoconductor drums301 and the transfer belt 310 at the transferring position equal foreach color at transferring time.

[0324] Similarly, it is impossible to take an accurate measurement atthe resist detecting positions if the individual color circumferentialspeeds do not coincide with each other. In particular, if the shiftspeed of the transfer belt 310 is used to compute a difference, thedifference between the circumferential speeds cannot be ignored.

[0325] For this reason, the optical scanner according to this embodimentcan set a distance between the transferring positions and the resistdetecting positions of the individual image forming stations, that is,

I=C+k·B, k=0, 1, 2, 3,

[0326] to an integral multiple of the circumferential length of thedriving roller 350. As a result, it is possible to make thecircumferential speed of the transfer belt 310 equal for each color atdetecting time.

[0327] Heretofore, the description has been focused on the case ofuniform difference of the resist positions. However, even if thetransfer belt 310 moves at periodically changing speeds due to speedvariations of the driving roller 350, it is possible to eliminate resistposition differences under the same configuration of the optical scanneraccording to the present invention.

[0328] In the above description, it has been demonstrated that adifference of resist positions that is regularly caused can besuppressed by designing arrangement. However, there may be a differenceof resist positions that is irregularly caused by load variations of thedriven rollers 351 and 352 that are used to tension the transfer belt310. According to the present invention, it is also possible toeffectively correct such an irregular difference of resist positions.

[0329] In this case, an amplitude and a frequency are detected byforming an encoder of the motor 353 coupled to the driving roller 350and a caterpillar pattern on the transfer belt 310. Then, the detectedamplitude and frequency are supplied to the gain adjusting part forcontrolling the movable mirrors 376 and 377 so that the movable mirrors376 and 377 can oscillate at a constant frequency. When phases areadjusted such that scanning positions have polarities inversed to thespeed variation, it is possible to cancel variations of resist positionsdue to the speed variation.

[0330] Here, it is possible to make the phases uniform by forming aresist mark at one point on the transfer belt 310.

[0331] A description will now be given, with reference to FIG. 27, of acorrection mechanism for correcting the slope and the curvature of ascanning line. FIG. 27 is an exploded perspective view of the correctionmechanism from the photoconductor drum 601 to the bottom surface of theoptical housing.

[0332] A toroidal lens 342 (343) is disposed on the bottom surface ofthe optical housing to face the photoconductor drum 301. Each of thetoroidal lenses are uniformly oriented with respect to optical axisdirection and the sub-scanning direction each other. A protrusion 348 bat the center of a box-shaped rib part 648 is coupled to a concave part503 of the optical housing. Regarding the main scanning direction(longitudinal axis direction), the optical axis direction of thetoroidal lens 342 is determined by placing the undersurfaces of flangeparts 346, which are provided at the both ends of the rib part 348, onreference surfaces 505 as illustrated in FIG. 27. Regarding thesub-scanning direction, one end of the rib part 348 is in contact with areference block part 506 in the optical housing. The other end of therib part 348 is in contact with a movable block part 508 coupled to astepping motor 507. The movable block part 508 can move parallel to thebottom surface of the optical housing. A blade spring 509 provides apressure in the optical axis direction and the sub-scanning direction.

[0333] A feed screw 510 is formed on the axis of the stepping motor 507so as to couple a D-shaped cylinder part 511 on the movable block part508. The D-shaped cylinder part 511 is inserted into a D-shaped hole 514of a motor support member 512 and is screwed to a perpendicular surface513. Thereby, it is possible to shift a block position of the movableblock part 508 against the toroidal lens 342 (343) by pulling or pushingthe D-shaped cylinder part 511 in the axis direction through rotation ofthe motor.

[0334] In this configuration, since the toroidal lens 342 (343) isrotated on a plane orthogonal to the optical axis by using the referenceblock side as a fulcrum and the slope of the focal line of the toroidallens 342 (343) is changed, it is possible to adjust the slope of ascanning line.

[0335] The toroidal lenses for forming the other color images exceptblack are configured to have the same mechanism by aligning thereference block sides thereof. Based on a detection result detected bythe detectors 360, the other color scanning lines are aligned parallelto each other with reference to the scanning line for black. In FIG. 27,mounting parts 523 of folding mirrors is provided on the bottom surfaceof the optical housing.

[0336] A description will now be given of a curvature correctionmechanism for correcting the curvature of a scanning line.

[0337] As shown in FIG. 27, a pair of a first metal plate 516 and asecond metal plate 517 are provided to the surface opposite to thesub-scanning directional block of the box-shaped rib part 348 of thetoroidal lens 342 (343). As shown in FIG. 27, the first metal plate 516is knob-shaped, and the second metal plate 517 has curved ends.

[0338] The first metal plate 516 is supported by catching bent parts518, which are provided to four corners thereof, to concave parts 348 aof the box-shaped rib part 348. An adjusting screw 521 is inserted intoa center hole 520, and thereby the first metal plate 516 is pulled tothe second metal plate 517 via the center hole 520. Then, since thecurved ends are in contact with the inner surfaces of a slope part 522,it is possible to adjust the slope angle of the slope part 522. Whencompressive force or tensile force is provided between the bent parts518 of the box-shaped rib part 348 with respect to the longitudinal axisdirection, it is possible to curve the focal line of the toroidal lens342 (343) in the sub-scanning direction.

[0339] In detail, when compressive force is provided, the toroidal lens342 (342) is concave toward the pair of the metal plates 516 and 517. Incontrast, when tensile force is provided, the toroidal lens 342 (343) isconvex toward the pair of the metal plates 516 and 517. For all colorsincluding black, the toroidal lenses are provided to have theabove-mentioned configuration. In this configuration, the toroidal lensis capable of correcting the curvature of a scanning line involved inobliquely incident optical beams to the polygon mirror 306. Furthermore,the toroidal lens is capable of also correcting the curvature of ascanning line due to placement errors of optical elements constitutingthe optical scanner. As a result, it is possible to improve linearity ofthe optical beams.

[0340] Here, when the detectors 360 detect a difference between resistpositions with respect to the main scanning direction or thesub-scanning direction, it is sufficient to correct the differencebetween resist positions just by correcting timings for writing images.Therefore, it is possible to superpose individual images recorded by theimage forming stations with high accuracy and form a high-quality fullcolor image without any color displacement.

[0341] In the above-mentioned embodiments, two movable mirror modules334 and 335 are provided as the optical axis adjusting part. However,the optical axis adjusting part may be configured as a single movablemirror module integrally including all movable mirrors corresponding tofour image forming stations.

[0342]FIG. 28 shows an exemplary single movable mirror module 550 thatintegrally accommodates four movable mirrors corresponding to four imageforming stations wherein the movable mirrors are arranged as ahound's-tooth pattern as illustrated.

[0343] In the above-mentioned embodiment, the optical scanner 300 inFIG. 17 is configured such that the polygon mirror 306 deflects opticalbeams in two directions by simultaneously using two surfaces thereof. Inan optical scanner according to this embodiment, a polygon mirror 551deflects optical beams by simultaneously using just one surface thereofas illustrated in FIG. 28.

[0344] Four optical beams from semiconductor lasers 552, 553, 554 and555 pass through coupling lenses 556, 557, 558 and 559, respectively,and then enter a synthesizing prism 560, which can be rotated on theoptical axis thereof by a predetermined angle. The optical beams enterone surface of the polygon mirror 551. In the vicinity of the surface ofthe polygon mirror 551, the optical beams cross each other with respectto the main scanning direction and travel at a predetermined pitchparallel to each other with respect to the sub-scanning direction. Acommon fθ lens 561 is prepared for the four optical beams. A pluralityof folding mirrors are disposed such that the four optical beams have anequal optical path length.

[0345] In the above-mentioned embodiments, the movable mirrors are usedas the optical axis adjusting part according to the present invention.However, the optical axis adjusting part is not limited to the movablemirrors. The optical axis adjusting part may comprise anything that canadjust the optical axis of an optical beam with respect to thesub-scanning direction. As shown in FIG. 29, a substrate 570 in whichliquid crystal is enclosed may be provided in an optical path between anilluminant and a polygon mirror. By applying an electric field to theliquid crystal and changing the orientation of the liquid crystal, anincident optical beam may be variably deflected. Specifically, adeflector as disclosed in Japanese Laid-Open Patent Application No.08-313941 can be used as such an optical axis adjusting part.

[0346]FIG. 30 shows an image forming part of an image forming apparatusaccording to one embodiment of the present invention wherein the imageforming part comprises an optical scanner, drum-shaped image carriers600K, 600C, 600M and 600Y (hereinafter referred to as photoconductordrums) and a transfer belt 605.

[0347] Referring to FIG. 30, each of the photoconductor drum 600K, 600C,600M and 600Y has processing members in accordance with anelectrophotographic process in the vicinity thereof. For example, theprocessing members are an electrifying part and a cleaning part forremoving remaining toners on a photoconductor drum after a transferringprocess. Additionally, for example, a paper feeding part is disposedbelow the undersurface of the transfer belt 605 to face thephotoconductors 600K, 600C, 600M and 600Y. The paper feeding partcomprises a paper feeding cassette to accommodate and supply recordingmedia (hereinafter referred to as record papers). Furthermore, atransferring part is disposed at the inner side of the transfer belt 605to face the photoconductor drums 600K, 600C, 600M and 600Y. A beltelectrifying part, is disposed at the upstream side of the rotationaldirection of the transfer belt 605, which is indicated by an arrow inFIG. 30. On the other hand, a belt separating charger, a fusing part andthe like are disposed at the downstream side of the rotational directionof the transfer belt 605. These parts are known to those skilled in theart and are configured according to FIG. 35 to be mentioned later. Forsimplicity, these parts are omitted in FIG. 30.

[0348] A description will now be given, with reference to FIG. 30, of atandem type color image forming apparatus for forming a color image. Inthis tandem type image forming apparatus, an optical scanner exposes theplurality of photoconductors 600K, 600C, 600M and 600Y so as to formindividual simple color latent images to be developed. After beingdeveloped, the resulting individual visible images on thephotoconductors 600K, 600C, 600M and 600Y are superposed and transferredonto the transfer belt 605 sequentially. Then, all the transferredimages on the transfer belt 605 are simultaneously transferred onto asame record paper so as to form a full color image.

[0349] Whenever a predetermined number of record papers are printed, theoptical scanner emits an optical beam, which is called a laser beam orscanning beam, to form a toner image for detecting color displacement.Based on three color displacement detecting toner images 631 on thetransfer belt 605 as illustrated in FIG. 15, a color displacementdetecting sensor 630 detects color displacement.

[0350] The optical scanner is configured as a unit such that a (notillustrated) box-shaped optical housing includes optical scanning imageforming systems that comprises an illuminant unit 610 to be mentionedlater, a polygon mirror 613, various lenses 612 and 614, mirrors 615M, awriting start position correcting part 611, beam spot position detectingparts 625 a and 625 b, which serve as position difference detectingparts. The optical scanner is disposed above photoconductors 600Y, 600M,600C and 600K. The writing start position correcting part 611 includes alead screw type actuator, which is driven by a stepping motor, to bementioned later as a rotation adjusting part for controlling awedge-shaped prism.

[0351] Here, the color displacement detecting sensor 630, which isinstalled in the image forming apparatus, is also used as the positiondifference detecting part to detect an amount to be corrected by thewriting start position correcting part 611. The color displacementdetecting sensor 630 may be used together with the beam spot positiondetecting parts 625 a and 625 b. In this case, the color displacementdetecting sensor 630 is used to roughly correct a position difference,and the beam spot position detecting parts 625 a and 625 b are used tofinely correct the position difference. Based on misalignment detectionresults, the writing start position correcting part 611 corrects andcontrols the position of an optical spot on a photoconductor duringwriting of image data.

[0352] The four photoconductor drums 600Y, 600M, 600C and 600K offersurfaces to be scanned by optical beams emitted from the opticalscanner. The photoconductor drums 600Y, 600M, 600C and 600K are linearlyarranged and rotationally driven. The optical scanner emits opticalbeams toward the individual photoconductor drums 600Y, 600M, 600C and600K so as to form latent images thereon. Then, the individual colorlatent images are developed as different color toner images on thephotoconductor drums 600Y, 600M, 600C and 600K. After the development,the resulting color toner images are sequentially superposed andtransferred onto the transfer belt 605, which serves as an intermediatetransferred member.

[0353] The optical scanner has four illuminant units 610 for emittingoptical beams to form different color latent images corresponding to theindividual photoconductor drums 600Y, 600M, 600C and 600K. Each of theilluminant units 610 at least includes a semiconductor laser 610 a and acollimate lens 610 b.

[0354] The optical beams from the four illuminant units 610 travel tothe polygon mirror 613, which serves as a deflecting part, through acylinder lens 612 and the writing start position correcting part 611.

[0355] After the optical beams are deflected by the polygon mirror 613,the deflected optical beams reach to the individual photoconductor drums600Y, 600M, 600C and 600K through the fθ lens 614, the folding mirrors615M and the toroidal lenses 620 so as to expose the photoconductordrums 600Y, 600M, 600C and 600K.

[0356] Each of the optical beams from the four illuminant units 610travels to the corresponding photoconductor drum via the opticalscanning systems from the cylinder lens 612 to the toroidal lens 620.Here, the polygon mirror 613 is directly coupled to a (not illustrated)polygon motor and is rotationally driven by the polygon motor.

[0357] The longitudinal axes of the photoconductor drums 600Y, 600M,600C and 600K correspond to the main scanning direction. The beam spotposition detecting parts 625 a and 625b are disposed to face each otherat both sides of outside areas out of an effective image area on thephdtoconductor drums 600Y, 600M, 600C and 600K. The beam spot positiondetecting parts 625 a and 625 b detect a writing start position and awriting end position, respectively.

[0358] In this embodiment, the optical scanner has at least onewedge-shaped prism on an optical path of an optical beam between theilluminant units 610 and the polygon mirror 613.

[0359] The writing position correcting part 611 shifts an optical spotwith respect to the sub-scanning direction by rotating the wedge-shapedprism approximately on the optical axis of the collimate lens 610 b.

[0360] A description will now be given, with reference to FIG. 31, of acorrection mechanism for correcting an optical spot with respect to thesub-scanning direction by using a wedge-shaped prism.

[0361] In FIG. 31, a wedge-shaped (trapezoidal) prism 1 is rotatedapproximately on the optical axis O-O of the collimate lens 610 b asindicated by the arrow 3. Thereby, it is possible to deflect an incidentoptical beam within the maximum deflection angle φ as indicated by thearrow 4 and adjust the position of the optical spot on a scanned surfacewith respect to the sub-scanning direction.

[0362] When the wedge-shaped prism 1 is rotated on the optical axis O-Oof the collimate lens 610 b as illustrated in FIG. 31, it is possible toadjust the deflection angle of the incident optical beam within themaximum deflection angle φ through deflection. Here, the maximumdeflection angle φ is represented by using, the following formula;

φ=(n−1)×α  (1),

[0363] where n is a refractive index of the wedge-shaped prism 1 and αis an apex angle of the wedge-shaped prism 1. Also, an amount ofcorrection P for a scanning position on a photoconductor is representedby using the following formula;

P=fc×β×tan φ×sinγ  (2),

[0364] where fc is a focal distance of the collimate lens 610 b, and βis a lateral magnification of all optical systems with respect to thesub-scanning direction. Here, it is preferable that the apex angle α[deg] of the wedge-shaped prism 1 satisfy the following inequality;

0.1<β×tan[(n−1)×α]<1.0  (3).

[0365] In the inequality (3), if the value of β×tan[(n−1)×α] is abovethe upper bound 1.0, wavefront aberration arises in the luminous flux.Thereby, the shape of the optical spot is deformed (occurrence of a sidelobe) or the diameter of the optical spot increases. In contrast, if thevalue is below the lower bound 0.1, the optical spot has insufficientsensitivity. Thereby, it is necessary to make the rotational anglegreater so as to adjust the writing start position. In this case, whenvariations over time should be corrected, the wedge-shaped prism 1cannot respond immediately.

[0366] When the apex angle α of the wedge-shaped prism 1 is properlyset, it is possible to realize the appropriate sensitivity. As a result,it is possible to prevent the excessive sensitivity unlike galvanometermirrors and reduce influences due to vibration. Therefore, it ispossible to position the optical spot with high accuracy.

[0367] In addition, the wedge-shaped prism 1 can be easily controlledbecause the wedge-shaped prism 1 is just rotationally driven. Thus, itis possible to flexibly correct the position of an optical spot duringwriting of image data compared to position correction for every batch atthe start-up time of the optical scanner or before printing operations.

[0368] A rotation adjusting part for the wedge-shaped prism 1 cancomprise a lead screw type actuator driven by a stepping motor. In thelead screw type actuator, the wedge-shaped prism 1 is mounted in a prismholder 5 as illustrated in FIG. 32. The prism holder 5 supports thewedge-shaped prism 1 such that the wedge-shaped prism 1 is freelyrotated on the optical axis O-O of the collimate lens 610 b. The prismholder 5 has an arm 5 a.

[0369] As shown in FIG. 32, an extensible spring 6 is provided betweenan end of the upper surface of the arm 5 a and a fixed member so thatthe arm 5 a is under pressure. Thereby, a clockwise moment with respectto the optical axis O-O is provided to the wedge-shaped prism 1 togetherwith the prism holder 5.

[0370] A receiving member 7, which is in contact with an end of thelower surface of the arm 5 a, obstructs rotation of the prism holder 5including the wedge-shaped prism 1. As shown in FIG. 32, the receivingmember 7 has a fundamentally column-shaped body and is cone-shaped atthe upper end with respect to the axis direction as indicated by thearrow. The tip of the cone is in contact with the lower surface of thearm 5 a. This contact point is referred to as an action point P of theactuator.

[0371] On the other hand, a nut 8 (or a female screw) is formed at theside of the receiving member 7 opposite to the tip so as to be coupledto the corresponding male screw integrally provided to the rotationalaxis of the stepping motor 9. This male screw is referred to as the leadscrew. The stepping motor 9 is coupled to the above-mentioned or anotherfixed member.

[0372] The above-mentioned rotating part is referred to as the leadscrew type actuator. The stepping motor 9 can rotate the prism holder 5together with the wedge-shaped prism 1 on the optical axis O-O of thecollimate lens 610 b.

[0373] Since the lead screw type actuator is driven by the steppingmotor 9, it is possible to control the rotational angle of thewedge-shaped prism 1 through a digital pulse signal applied to thestepping motor 9. As a result, after an amount of misalignment iscomputed by some computing part such as a microcomputer, it is possibleto easily the rotation of the wedge-shaped prism 1 through feedbackcontrols by providing an appropriate pulse signal.

[0374] The lead screw type actuator, which serves as the rotationadjusting part for the wedge-shaped prism 1, can be installed in theoptical scanner as the writing starting position correcting part 611.

[0375] A description will now be given of another correction mechanismfor correcting a color difference with respect to the sub-scanningdirection.

[0376] In the color image forming apparatus shown in FIG. 30 or anothercolor image forming apparatus described later with reference to FIG. 35,a color difference can be corrected with respect to the sub-scanningdirection in such a way that the positions of scanning lines, which areemitted from optical scanning systems corresponding to individualcolors, are properly adjusted to make the color difference approximatelyzero when individual color images are superposed on an intermediatetransferred member.

[0377] When each of four optical beams corresponding to the four colorsis attempted to be placed at a predetermined position in order to adjustthe writing start position of the optical beam, there is a risk thatoptical performance such as the diameter of the optical spot is degradeddue to an erroneous increase in rotational eccentricity. In addition,since more components are required for the writing start positioncorrecting part, the fabrication cost is increased.

[0378] In order to eliminate this problem, the optical scanner accordingto the present invention sets one of the four colors: yellow (Y),magenta (M), cyan (C) and black (K) as a reference color and correctsthe scanning positions of the other color optical beams relative to thescanning position of the reference color optical beam such that thescanning positions of the other color optical beams is approximatelyequal to the scanning position of the reference color. On the otherwords, an image of high reproducibility, in a sense that hue variationsare sufficiently suppressed, can be obtained through the correction of“relative color displacement”.

[0379] In this case, it is sufficient that wedge-shaped prisms 1 forcorrecting writing start positions are prepared for three of thescanning beams for the four colors Y, M, C and K. Consequently, justthree wedge-shaped prisms 1 should be provided corresponding to thethree scanning beams, and therefore just three writing start positioncorrecting parts 611 are required.

[0380] In the optical scanner in FIG. 30, the three writing startposition correcting parts 611 are installed for the three colors otherthan the reference color. The writing start position correcting part 611for the reference color comprises a parallel flat plate or no component,that is, air. The writing start position correcting parts 611 for theother colors are formed as units that integrally include a wedge-shapedprism 1 and other components.

[0381] According to the above-mentioned optical scanner, since lessportions and less amounts are adjusted, it is possible to easily correcta relative color difference even if scanning lines for the other colorsare greatly misaligned to that of the reference color. In addition, itis possible to correct a color difference less than one line. Fromexperiences of the inventors, if the relative color displacement isdecreased below 30 μm, it is possible to obtain an image whose colordisplacement can be substantially ignored.

[0382] In the optical scanner according to the present invention, blackis set as the reference color. In general, arbitrary color images can beformed of combinations of the three colors Y, M and C. In order toimprove sharpness of the color images and enhance resolutions ofcharacter images, however, it is common that the optical scanner has ablack process.

[0383] When the optical scanner adopts black as the reference color, theoptical scanner has the following advantages. Since black has a highercontrast than the other colors, a black portion of an image is highlysensitive to variations of the diameter and the position of an opticalspot due to external factors such as vibration and temperature changes.For this reason, if black is set as the reference color, it is possibleto enhance the rigidity of optical components of the optical scanningsystem for black and thereby realize the tolerant optical scanningsystem to the external factors.

[0384] As mentioned above, since scanning lines for three of the fourcolors should be adjusted, it is sufficient that three wedge-shapedprisms 1 are prepared. In other words, it is possible to obtain an imageof high color reproducibility in which hue variations are satisfactorilysuppressed. In this case, just three writing start position correctingparts 611 should be prepared, for example, for the optical scanner inFIG. 30.

[0385] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0386] The optical scanner comprises at least one wedge-shaped prism 1,a writing start position correcting part 611 and a position differencedetecting part. The wedge-shaped prism 1 is disposed in an optical pathbetween the illuminant unit 610 and the polygon mirror 613. The writingstart position correcting part 611 rotates the wedge-shaped prism 1approximately on the optical axis so as to make the position of anoptical spot changeable with respect to the sub-scanning direction. Theposition difference detecting part detects relative sub-scanningdirectional differences between writing start positions on thephotoconductor drums 600Y, 600M, 600C and 600K. Based on data regardingthe differences detected by the position difference detecting part, theoptical scanner is configured to feedback-control the writing startposition correcting part 610.

[0387] If the position difference detecting part has the configurationas illustrated in FIG. 33, it is possible to use the position differencedetecting part together with the beam spot position detecting part 625 afor detecting the writing start positions or the beam spot positiondetecting part 625b for detecting the writing end positions.

[0388] According to the above-mentioned optical scanner, it is possibleto effectively correct a difference between the writing start positionswith respect to the sub-scanning direction on a photoconductor drum dueto variations over time. Here, it is noted that a stepping motor, anultrasonic motor or the like can be used to adjust rotation of thewedge-shaped prism 1 through the feedback control, as mentioned withreference to FIG. 32.

[0389] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0390] The optical scanner is configured to use the writing startposition correcting part 611 to perform the correction process at leastonce since start time of a printing process based on data regardingposition differences detected by the position difference detecting part.

[0391] According to the above-mentioned optical scanner, even if asignificant color displacement occurs over time regardless of properadjustment of the optical scanner at shipment, it is possible toproperly correct the color displacement due to periodical variations(places and seasons) over time because the correction of positiondifferences is performed at least once before a printing process.

[0392] The position difference detecting part may be configured fromnon-parallel PD (Photodiode) sensors as illustrated in FIG. 33 or a beamspot position detecting part 625 using the non-parallel PD sensor.

[0393] Alternatively, the following prevailing correction method may beadopted. Specifically, before printing a first sheet, a reference tonerimage 631 is formed on the transfer belt 605 to detect a colordisplacement. A color displacement detecting sensor 630 is used todetect an amount of the color displacement. The color displacementdetecting sensor 630 is formed of a reflection or transmission typesensor, which includes LED (Light Emitting Diode) and monitors for lightintensity by means of a PD, and serves as the position differencedetecting part installed in an image forming apparatus. Based on thedetection, an amount of the correction is determined for a differencebetween writing start positions of the individual photoconductor drums600Y, 600M, 600C and 600K. Finally, the writing start positioncorrecting part 611, which serves as the rotation adjusting mechanismfor the wedge-shaped prism 1, receives feedback.

[0394] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0395] As shown in FIG. 34, when a plurality of record sheets aresuccessively printed out (during the interval designated by the notationB), heat is generated from the polygon mirror 613 and semiconductorlaser 610 a in the interior of the optical scanner and drastictemperature variations occur between the interval A of a start periodand the interval B. In the exterior of the optical scanner, on the otherhand, drastic temperature variations occur in the interior of the imageforming apparatus due to a heater for fusing toners. In this case,optical spots are also variably positioned on photoconductor drums. Forthis reason, there is a problem in that output color images have greatlydifferent hues depending on the number of output sheets.

[0396] In the optical scanner according to this embodiment, timing ofthe feedback control is determined such that the correction is performedwithin a time interval between printed sheets. A control time intervalT_(A) (between a time when an instruction for correcting a positiondifference is provided and a finish time of the control) is determinedto satisfy the following inequality;

T _(A)<0.8×(D/V)  (4),

[0397] where D is a distance between the sheets, and V is the linearspeed of a photoconductor drum (linear speed of an image carrier).

[0398] If the wedge-shaped prism 1 is rotated at a response speed thatmeets the inequality (4), it is possible to form proper color images,each of which has a same hue as each other, even during drastictemperature variations due to successive printing.

[0399] However, if the control time interval T_(A) is above the upperbound of the inequality (4), it is impossible to perform the correctionduring the successive printing. In this case, it is necessary totemporarily halt the printing process in order to correct scanninglines. As a result, although the tandem type color image formingapparatus has an advantage of high-speed printing, the tandem type colorimage forming apparatus cannot satisfactorily exert the advantage.

[0400] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0401] In the optical scanner, a detection time interval T_(S) (betweena start time for correcting a position difference and a finish time ofdetection) is determined to satisfy the following inequality;

T _(S)<10×(L/V)  (5),

[0402] where L is a length of a printed sheet with respect to the outputdirection, and V is the linear speed of a photoconductor drum.

[0403] The inequality (5) implies that when a position differencebetween scanning lines is detected during printing at least five sheets,hue variations is invisible to naked eyes even if drastic temperaturevariations occur in the image forming apparatus.

[0404] However, if the detection time interval T_(S) is above the upperbound of the inequality (5), there is a risk that the resulting hue isproblematic. Here, it is noted that the detection time interval T_(S)includes a computation time for computing the correction amount for theposition difference. The computation time is a time interval required toperform an averaging process for the purpose of noise reduction, improvethe accuracy for position detection such as processing of an abnormalvalue and compute the correction amount to be supplied to the scanningline correcting part for feedback.

[0405] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0406] In this optical scanner, the position difference detecting partdetects the position of an optical spot by using non-parallel PD sensorsdisposed out of an effective writing area of the optical scanner withrespect to the main scanning direction.

[0407] As shown in FIG. 33, the receiving surface of the photodiode PD1(PD1′) is orthogonal to a scanning beam, and the receiving surface ofthe photodiode PD2 (PD2′) is inclined to that of PD1 (PD1′). This slopeangle is designated by α1. Also, the scanning beam is referred to as L1and L2 before and after a temperature variation due to a heater,respectively. Here, it is assumed that a position difference ΔZ(unknown) arises with respect to the sub-scanning direction. Here,passage times T1 and T2 of the respective scanning beams L1 and L2between a pair of non-parallel photodiodes, for example, between PD1 andPD2 (or PD1′ and PD2′) is measured. Based on the results, a timedifference T2-T1 is found, and then the sub-scanning directionalscanning position (the writing start position) of the scanning beam ismonitored for and detected.

[0408] It is possible to easily calculate a difference between dotpositions with respect to the sub-scanning direction (=the sub-scanningdirectional correction amount ΔZ) from the slope angle α1 between thereceiving surfaces of PD1 and PD2 and the known time difference T2-T1.The optical scanner is configured to correct the correction amount ΔZ byusing the writing start position correcting part 611.

[0409] According to the above-mentioned optical scanner, when aplurality of record sheets are successively printed out, it is possibleto correct optical spots on photoconductor drums during writing of theimages even if the optical spots are variably positioned on thephotoconductor drums due to temperature variations.

[0410] Here, magnification variations with respect to the main scanningdirection may be monitored for by detecting variations of a timeinterval T0 required for the scanning beam to pass between thephotodiodes PD1′ and PD1.

[0411] A description will now be given of a tandem type color imageforming apparatus according to an embodiment of the present invention.

[0412] As shown in FIG. 34, when a plurality of image sheets aresuccessively printed out, heat is generated from a polygon motor (notillustrated) for driving the polygon mirror 613 and semiconductor lasers(illuminants) 610 a in the optical scanner in the image formingapparatus. In the exterior of the optical scanner, on the other hand,drastic temperature variations of the interior of the image formingapparatus occur when a heater, which serves as a fusing part, fusestoners. In this case, optical spots are also variably positioned onphotoconductor drums, resulting in the problem that output color imageshave hues more different from each other as the number of the outputcolor images increases.

[0413] The tandem type color image forming apparatus according to thisembodiment is configured to have an optical scanner that comprises theposition difference detecting part, at least one wedge-shaped prism anda writing start position correcting part 611 so as to adjust thepositions of optical spots on photoconductor drums during writing ofimages. The position difference detecting part, which also serves as abeam spot position detecting part, detects position differences betweenscanning beams on the individual photoconductor drums 600Y, 600M 600Cand 600K, as illustrated in FIG. 30. The wedge-shaped prism is disposedin an optical path between an illuminant and a deflecting part. Thewriting start position correcting part 611 rotates the wedge-shapedprism approximately on the optical axis so as to shift the position ofthe beam spot with respect to the sub-scanning direction. Based on thedetected position differences measured by the position differencedetecting part, the optical scanner can control the positions of opticalspots on the photoconductor drums during writing of the images.

[0414] The exemplary color image forming apparatus that has the opticalscanner according to the present invention has been described withreference to FIG. 30. In the following, a tandem type color imageforming apparatus according to another embodiment of the presentinvention is described with reference to FIG. 35.

[0415]FIG. 35 roughly shows the structure of the tandem type color imageforming apparatus. Referring to FIG. 35, in this tandem type color imageforming apparatus, an optical scanner exposes a plurality ofphotoconductor drums (image carriers) 700Y, 700M, 700C and 700K so as toform electrostatic latent images. After the electrostatic latent imagesare developed, the resulting visible images on the individualphotoconductor drums 700Y, 700M, 700C and 700K are superposed andtransferred onto a transfer belt 710. Then, the images on the transferbelt 710 are entirely transferred onto a sheet-shaped medium (recordpaper) so as to obtain a color image.

[0416] In FIG. 35, the transfer belt 710 serves as an intermediatetransfer belt for carrying a transferred paper (not illustrated) from apaper cassette 720. The transfer belt 710 is horizontally disposed atthe inner-lower side of the image forming apparatus. From the upstreamof the carrying direction of the transfer belt 710, the yellowphotoconductor 700Y, the magenta photoconductor 700M, the cyanphotoconductor 700C and the black photoconductor 700K, in the order, arearranged to have an equal interval between adjacent photoconductordrums.

[0417] In the following, suffixes Y, M, C and K may be attached toreference numerals corresponding to the four colors: yellow, magenta,cyan and black, respectively. All the photoconductor drums 700Y, 700M,700C and 700K are formed to have a same diameter, and various processingmembers for forming a color image are provided in accordance with anelectrophotographing process.

[0418] For instance, the photoconductor 700Y is focused hereinafter. Inthe vicinity of the photoconductor drum 700Y, an electrifying charger701Y, an optical scanning image forming system 702Y, a developing device703Y, a transferring charger 704Y and a cleaning device 705Y aredisposed. The electrifying charger 701Y serves as an electrifying part.The developing device 703Y serves as a developing part. The transferringcharger 704Y serves as a transferring part. The cleaning device 705Yserves as a cleaning means. In the vicinity of each of the otherphotoconductor drums 700M, 700C and 700K, the corresponding parts areprovided similarly to the photoconductor drum 700Y. In this tandem typecolor image forming apparatus, the photoconductor drums 700Y, 700M, 700Cand 700K are exposed in accordance with individual settings. A pluralityof optical scanning image forming systems 702Y, 702M, 702C and 702K areprovided corresponding to the photoconductor drums 700Y, 700M, 700C and700K, respectively.

[0419] In the vicinity of the transfer belt 710, a pair of resistrollers 721, and a charger 722, which serves as a belt electrifyingpart, are disposed in the upstream side of the carrying direction of thetransfer belt 710 as indicated by the arrow in FIG. 35. On the otherhand, a belt detaching charger 723, a de-electrifying charger 724 and acleaning device 725 are provided at the downstream side of the carryingdirection of the transfer belt 710. Furthermore, a fusing device 726,which serves as q fusing part, is provided in the downstream side of thecarrying direction from the belt detaching charger 723. The fusingdevice 726 is coupled to an output tray 728 via a pair of output rollers727. In the above description, the suffix Y is omitted for simplicity.

[0420] In such a configuration, for example, when a full-color mode(multi-color mode) is selected, electrostatic latent images are formedon the photoconductors 700Y, 700M, 700C and 700K by optical beams fromthe individual scanning image forming optical systems based on imagesignals for individual colors: Y, M, C and K.

[0421] These electrostatic latent images are developed by the individualdeveloping apparatuses for the color toners so as to form toner images.The toner images are sequentially transferred and superposed onto atransferred paper that are electrostatically absorbed on the transferbelt 710 to be carried. Then, the resulting paper is fused as afull-color image and is supplied to the output trey 728.

[0422] On the other hand, when a monochrome mode (single-color mode) isselected, the photoconductors 700Y, 700M and 700C and associatedprocessing members are made inactivated, an electrostatic latent imageis formed on only the photoconductor 700K by an optical beam from thescanning image forming optical system 702K based on an image signal forblack. This electrostatic latent image is developed by black toners soas to form a toner image. The toner image is transferred onto atransferred paper that is electrostatically absorbed to be carried. Theresulting paper is fused as a monochrome image and is supplied to theoutput trey 278.

[0423] Here, two fθ lenses 735M1 and 735M2 are fixed to a plate 745M inan optical housing 750. The fθ lenses 735M1 and 735M2 are fully orpartially in contact with the corresponding surface of the plate 745M.The fθ lenses 735M1 and 735M2 are formed of a reasonable plasticmaterial that can be aspherically-shaped with ease. Specifically, asynthetic resin is preferably used because of its low water absorbingproperty, high transparency and good formability. In this embodiment,the polygon mirror comprises an upper mirror 730U and a lower mirror730D.

[0424] This color image forming apparatus has the above-mentionedoptical scanner in the optical housing 750. Although not illustrated inFIG. 35, the color displacement detecting sensor 330 as illustrated inFIG. 30 may be used as the position difference detecting part.

[0425] While a large number of color images are successively printedout, rapid temperature variations are caused, especially, by heat of apolygon motor in the optical scanner. For this reason, if a firstprinted color image is compared to another color image that is printedout after several sheets have been printed out, there arises a problemthat variations on color tone can be observed. If the optical scannerincludes a wedge-shaped prism between the deflecting part and thescanned surface to be exposed, it is possible to not only correctdifferences of scanning positions and set the scanning position withhigh accuracy. In this case, furthermore, even if the temperature israpidly varied, especially, due to successive printing, it is possibleto obtain a satisfactory color image with little color displacement.Additionally, the above-mentioned optical scanners according to theembodiments of the present invention have their own merits.

[0426] A description will now be given of an optical scanner accordingto one embodiment of the present invention.

[0427] In the above-mentioned optical scanner in FIG. 30, a positiondifference between optical spots on the photoconductor drums 600Y, 600M,600C and 600K can be corrected with respect to the sub-scanningdirection based on position difference data that have been recorded inadvance or position difference data that are detected by the positiondifference detecting part. This correction is conducted by the writingstart position correcting part.

[0428] When the wedge-shaped prism is used in the writing start positioncorrecting part, the optical scanner has the following advantages. Therotation of the wedge-shaped prism 1 can be easily controlled. Thus, itis possible to correct the positions of optical spots even duringwriting of image data relative to batch-based position correction thatis performed intensively at starting-up time of the image formingapparatus or immediately before printing out. Even when the temperatureis drastically varied during the starting-up interval A and thesuccessive printing interval B as shown in FIG. 34 or a positiondifference occurs due to the unsteady speed of an intermediatetransferring member and a photoconductor drum, the position differencecan be eliminated in real-time based on data regarding positiondifferences obtained by the position difference detecting part.Therefore, it is possible to effectively suppress occurrence of theposition difference.

[0429] In FIG. 31, if the angle α of the wedge-shaped prism 1 isappropriately set, it is possible to realize appropriate sensitivity.Thus, it is possible to set the positions of optical spots with highaccuracy without too-high sensitivity as a galvanometer mirror orinfluences due to vibration.

[0430] When a material and a shape of the wedge-shaped prism 1 and theprism holder 2 is appropriately selected, for example, the wedge-shapedprism 1 and the prism holder 2 are formed of a resin and have thinbodies, it is possible to decrease the weights thereof. For this reason,the high response speed of the wedge-shaped prism 1 makes it possible tocorrect a position difference at a higher frequency than conventionalcorrection methods in which the positions of optical spots are correctedby tilting/shifting relatively heavy optical elements such as a longfolding mirror, a scanning lens, a roof mirror and an illuminant unit.

[0431] When the wedge-shaped prism 1 is used to correct the positions ofoptical beams, it is possible to maintain appropriate positions ofoptical spots even at power OFF compared to conventional correctionmethods in which the positions of optical spots are corrected byadjusting a voltage applied to a liquid crystal element and an electricoptical element such as PLZT. Furthermore, it is possible to implementthe above-mentioned correction at a reasonable cost.

[0432] From inventors' experience, when an amount of displacementbetween colors is less than or equal to 30 μm, it is possible to achievea condition in which color displacement is almost never noticeable inpractice. According to the above-mentioned embodiment, it is possible torealize such a condition.

[0433] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0434] When the wedge-shaped prism 1 is rotated on the optical axis O-Oof the collimate lens 610 b as illustrated in FIG. 30, it is possible tomake the deflection angle variable within the maximum deflection angleφ. Here, the maximum deflection angle φ can be represented by thefollowing formula (6);

φ=(n−1)×α  (6),

[0435] where n is a refractive index of the wedge-shaped prism 1 and αis an apex angle of the wedge-shaped prism 1.

[0436] Also, the correction amount ΔZ with respect to the sub-scanningdirection on a photoconductor is represented by the following formula(7);

ΔZ=Δγ|m×fc×(n−1)×α|  (7),

[0437] where Δγ is an adjusted angle of the wedge-shaped prism 1 on theoptical axis O-O, m is a sub-scanning directional lateral magnificationof all optical systems between an illuminant and a scanned surface, andfc is a focal distance of the collimate lens 610 b.

[0438] In the optical scanner according to this embodiment, amisalignment of an beam spot is corrected by setting the apex angle α[deg] of the wedge-shaped prism 1 to satisfy the following inequality(8);

1<|m×fc×(n−1)×α|<30   (8).

[0439] If the value of |m×fc×(n−1)×α| is above the upper bound of theinequality (8), the optical spot is irregularly shaped (generation of aside lobe) or the diameter of the optical spot increases due towavefront aberration of the luminous flux. In contrast, if the absolutevalue is below the lower bound of the inequality (8), it is necessary tomake the rotation angle greater in order to improve insufficientsensitivity and adjust the writing start position. As a result, itbecomes difficult to realize a high-speed response when variations overtime are to be corrected.

[0440] Therefore, if the inequality (8) is met, it is possible toeliminate the above-mentioned problems, that is, enlargement of a beamspot diameter, insufficient sensitivity to the correction, a largerotation angle for adjusting the writing start position, and aninsufficient response speed to correction for variations over time.Here, the rotation of the wedge-shaped prism 1 can be easily controlledby using a stepping motor or an ultrasonic motor as the drive source.

[0441] A description will now be given of an image forming apparatusaccording to another embodiment of the present invention.

[0442] Although an intermediate transferred body is configured as thetransfer belt 605 in the image forming apparatus illustrated in FIG. 30,the intermediate transferred body may be drum-shaped. FIG. 36A shows avariation of the position of an optical spot with respect to thesub-scanning direction on such a drum-shaped intermediate transferredbody and a variation of the position of an optical spot with respect tothe sub-scanning direction on a belt-shaped intermediate transferredbody.

[0443] As seen from FIG. 36A, the difference ΔZ of optical spotpositions with respect to the sub-scanning direction, that is, thesub-scanning directional correction amount as previously mentioned, isperiodically caused on the intermediate transferred bodies. In FIG. 36A,one period corresponds to a time interval T_(m) necessary for onerotation of the intermediate transferred bodies. In an intermediatetransferred body is formed as a belt-shaped or drum-shaped rotationalbody, the time interval T_(m) is represented by the following formula;

T _(m) L _(m) /v _(m)  (9),

[0444] where L_(m) is a length of one rotation of the intermediatetransferred body, and v_(m) is a linear velocity of the intermediatetransferred body.

[0445] It is preferable that the time interval T_(m) satisfy thefollowing inequality;

0.5<T _(m)(=L _(m) /v _(m))<5[sec]  (10).

[0446] In the inequality (10), if the time interval T_(m) is above theupper bound, the intermediate transferred body is highly sensitive toexogenous factors such as vibration because of a too large length of oneperiod. In contrast, if the time interval T_(m) is below the lowerbound, the wedge-shaped prism 1 cannot follow a high response speednecessary for the position correction for an otpical spot. Therefore,when the time interval T_(m) satisfies the inequality (10), it ispossible to eliminate the above-mentioned problems on the intermediatetransferred body, that is, less sensitivity to exogenous factors such asvibration, and follow-up capability toward the position correction foran optical spot.

[0447]FIG. 36B shows a position difference of the position of an opticalspot with respect to the sub-scanning direction after the optical spotposition is corrected. When the misalignment of an optical spot positionon the intermediate transferred body is corrected by adjusting theoptical spot position, it is possible to properly correct alow-frequency component of the misalignment. According to thecorrection, however, it is impossible to correct an extremelyhigh-frequency component of the misalignment.

[0448] A description will now be given of an optical scanner accordingto one embodiment of the present invention.

[0449] In the above-mentioned writing start position correcting part inFIG. 31, a shift amount Δx of the linear actuator (=a displacementamount of a nut=a displacement amount of an action point P) isrepresented by the following formula;

Δx=R×tan(Δγ)  (11),

[0450] where R is a distance between the rotational center and theaction point of the actuator, and Δγ is a rotational angle of thewedge-shaped prism 1.

[0451] On the other hand, the time interval T_(m) necessary for onerotation of the intermediate transferred body is represented, asmentioned previously, by the formula (9);

T _(m) =L _(m) /v _(m) [sec].

[0452] Thus, a drive frequency N per unit time of the stepping motorrequired to control the sub-scanning directional correction amount ΔZ ona photoconductor is represented by the following formula;

N=Δx/p×N ₀ /T _(m)  (12),

[0453] where p is a screw pitch of the lead screw, and N₀ is the numberof pulses per one rotation of the stepping motor.

[0454] When the stepping motor satisfies the following inequality;

10<N<2000[pps]  (13),

[0455] it is possible to properly reduce color displacement.

[0456] In the inequality (13), if N is above the upper bound (2000 [pps]or preferably 1000 [pps]), the stepping motor cannot respond and thecorrection for misalignment of a beam spot cannot be followed. Incontrast, if N is below the lower bound, the position of a beam spotcannot be corrected with insufficient accuracy because of roughresolution.

[0457] On the other hand, a torque (rotational moment) T of the steppingmotor is represented by the following formula; $\begin{matrix}{{T = \frac{T_{1}p}{2\pi \quad R}},} & (14)\end{matrix}$

[0458] where T₁ is a torque generated by tension of the spring, p is ascrew pitch of the lead screw, and R is a distance between therotational center and the action point.

[0459] The maximum number N_(max) of response pulses of the steppingmotor is obtained from a pull-in drive frequency toward the torque Twith reference to the characteristic diagram of the stepping motor asillustrated in FIG. 37.

[0460] Thus, the number of pulses N, represented by the formula (12),per unit time required to control the sub-scanning directionalcorrection amount ΔZ on a photoconductor needs to satisfy the followinginequality;

N<N _(max)  (15).

[0461] A description will now be given of an optical scanner accordingto another embodiment of the present invention.

[0462] As shown in FIG. 15, the optical scanner includes a misalignmentdetecting part, at least one wedge-shaped prism 1 and a writing startingposition correcting part 140. The misalignment detecting part, whichalso serves as a beam spot position detecting part, detects a positiondifference with respect to the sub-scanning direction between scanningbeams on the individual photoconductors 160Y, 160M, 160C and 160K. Thewriting starting position correcting part 140, adjusts positions of thebeam spots with respect to the sub-scanning direction by rotating thewedge-shaped prism 1 approximately on the optical axis. Based onmisalignment data measured by the misalignment detecting part, thepositions of the beam spots on the photoconductors are controlled duringwriting of image data.

[0463] The position difference detecting part detects a position of abeam spot by using a non-parallel photo diode sensor (PD) provided outof a written area with respect to the main scanning direction of theoptical scanner. It is preferable that the non-parallel photo diodesensor be provided out of an effective writing area of a scanning beamfor each of the photoconductors 160Y, 160M, 160C and 160K, for example,as the beam spot position detecting parts 300 a and 300 b in FIG. 15. Atthis time, the misalignment detecting part may also detect asynchronizing signal for determining the writing starting position withrespect to the main scanning direction. In this embodiment, themisalignment detecting part detects the synchronizing signal fordetermining the writing starting position.

[0464] In the position difference detecting part, which is implementedas the beam spot position detecting parts 625 a and 625 b, a detectiontime interval T_(S) for the scanning beams L1 and L2 by the non-parallelphoto diodes PD1 and PD2, that is, the time interval between thestarting time and the finishing time of the misalignment detection, ischaracterized in that the detection time interval T_(S) satisfies thefollowing inequality;

T _(S)<10×(L _(p) /v _(p))  (16),

[0465] where L_(p) is a length of a recorded paper with respect to theoutput direction (the shift direction of the transfer belt 605 indicatedby the arrow in FIG. 30), and V_(p) is a linear velocity of aphotoconductor.

[0466] The inequality (16) implies that even if drastic temperaturevariations occur, it is possible to make color tone variations invisibleto the naked eyes by detecting misalignment of a scanning line while atleast five sheets are being printed out. If the detection time intervalT_(S) is above the upper bound, there is a risk that a problematic colortone may be generated.

[0467] Here, the detection time interval T_(S) includes a time intervalfor computing a correction amount of misalignment. This computationincludes a computation time during which a correction amount forfeedback to the scanning line correcting part is computed so as toimprove accuracy for misalignment detection such as averaging andabnormal value processing for the purpose of noise reduction.

[0468] A description will now be given of exemplary numerals accordingto the present invention.

[0469] (1) Condition

[0470] Performance of optical systems m=9.4, fc=15 [mm]

[0471] Wedge-shaped prism α=2°, n=1.51 (BK7)

[0472] Lead screw type actuator T₁=25×10⁻³ [Nm], p=0.3 [mm], R=16 [mm],N₀=20 [pulse/one rotation]

[0473] Intermediate transferred body L_(m)=500 [mm], v_(m)=250 [mm/s]

[0474] (2) Example of computation|m×fc×(n−1)×α|=|9.4×15×(1.51−1)×(2°/180°×π)|=2.5, which satisfies theinequality (8).${{\Delta \quad \gamma} = {\frac{\Delta \quad Z}{{m \times {fc} \times \left( {n - 1} \right) \times \alpha}} = {{0.2/2.5} = {0.2\left( {= 11^{{^\circ}}} \right)}}}},$

[0475] Δx=R×tan(Δγ)=16×tan(0.2)=3.2 [mm], and T_(m)=L_(m)/v_(m)=2 [sec],resulting in N=Δx/p×N₀/T_(m)=3.2/0.3×20/2=106 [pps], which satisfies theinequality (13).${T = {\frac{T_{1}p}{2\pi \quad R} = {{25 \times 10^{- 3} \times {0.3/\left( {2\pi \times 16} \right)}} = {7.46 \times {10^{{- 3}\quad}\lbrack{Nm}\rbrack}}}}},$

[0476] which implies that a stepping motor that satisfies the inequalityN_(max)>N is used for N_(max) in the motor characteristic diagramsimilar to FIG. 37.

[0477] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope of the present invention.

[0478] The present application is based on Japanese priorityapplications No. 2002-193652 filed Jul. 2, 2002, No. 2002-276311 filedSep. 20, 2002, No. 2002-274073 filed Sep. 19, 2002, and No. 2002-274075filed Sep. 19, 2002, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. An optical scanner for scanning an image carrier,comprising: an illuminant part emitting an optical beam; a deflectingpart deflecting the optical beam; an image forming part focusing thedeflected optical beam on the image carrier; and an optical axisadjusting part being provided between the illuminant part and thedeflecting part, said optical axis adjusting part adjusting a beam spotposition of the optical beam on the image carrier with respect to asub-scanning direction.
 2. The optical scanner as claimed in claim 1,wherein the optical axis adjusting part comprises a movable mirror. 3.The optical scanner as claimed in claim 2, wherein the movable mirrorhas at least one vibration mode in which said movable mirror vibrateswith respect to the sub-scanning direction.
 4. The optical scanner asclaimed in claim 1, wherein the optical axis adjusting part slightlyvibrates the beam spot position of the optical beam on the image carrierwith respect to the sub-scanning direction slowly relative to a periodof optical scanning.
 5. The optical scanner as claimed in claim 4,wherein the optical axis adjusting part comprises a phase adjusting partadjusting a phase of vibration.
 6. The optical scanner as claimed inclaim 4, wherein the optical axis adjusting part comprises an amplitudeadjusting part adjusting an amplitude of vibration.
 7. An image formingapparatus, comprising: a plurality of image carriers; an optical scannerforming latent images on the plurality of image carriers, said opticalscanner comprising: an illuminant part comprising a plurality ofilluminants, said plurality of illuminants emitting optical beams; adeflecting part deflecting the optical beams; an image forming partfocusing the deflected optical beams on the plurality of image carriers;and an optical axis adjusting part being provided between the illuminantpart and the deflecting part, said optical axis adjusting part adjustingbeam spot positions of the optical beams on the plurality of imagecarriers with respect to a sub-scanning direction; a developing partdeveloping the latent images so as to form visible images; and atransferred member onto which the visible images are transferred fromthe plurality of image carriers, wherein the optical axis adjusting partcomprises a movable mirror and slightly vibrates the beam spot positionsof the optical beams on the image carriers with respect to thesub-scanning direction slowly relative to a period of optical scanning.8. The image forming apparatus as claimed in claim 7, wherein theoptical axis adjusting part slightly adjusts rotational time of theimage carriers from writing positions onto the image carriers to atransferring position onto the transferred member while the latentimages are formed on the plurality of image carriers.
 9. The imageforming apparatus as claimed in claim 8, wherein the illuminant partcomprises a selecting part selecting one of the plurality of illuminantsfor an optical beam that optically scans a head line with respect to thesub-scanning direction from which the latent images are formedcorresponding to the rotational time of the image carriers from thewriting positions onto the image carriers to the transferring positiononto the transferred member.
 10. The optical scanner as claimed in claim1, wherein the optical axis adjusting part comprises a wedge-shapedprism.
 11. The optical scanner as claimed in claim 10, wherein theoptical axis adjusting part adjusts the beam spot position of theeoptical beam with respect to the sub-scanning direction by rotating thewedge-shaped prism approximately on an optical axis.
 12. The opticalscanner as claimed in claim 11, wherein the optical axis adjusting partadjusts the beam spot position of the optical beam during writing of animage.
 13. An image forming apparatus, comprising: a plurality of imagecarriers; an optical scanner forming latent images on the plurality ofimage carriers, said optical scanner comprising: an illuminant partcomprising a plurality of illuminants, said plurality of illuminantsemitting optical beams; a deflecting part deflecting the optical beams;an image forming part focusing the deflected optical beams on theplurality of image carriers; and an optical axis adjusting part beingprovided between the illuminant part and the deflecting part, saidoptical axis adjusting part adjusting beam spot positions of the opticalbeams on the plurality of image carriers with respect to a sub-scanningdirection; a developing part developing the latent images so as to formvisible images; and a transferred member onto which the visible imagesare transferred from the plurality of image carriers, wherein theoptical axis adjusting part comprises a wedge-shaped prism and adjuststhe beam spot positions of the optical beams with respect to thesub-scanning direction by rotating the wedge-shaped prism approximatelyon an optical axis.
 14. The image forming apparatus as claimed in claim13, further comprising a position difference detecting part detecting adifference between the beam spot positions of the optical beams on theplurality of image carriers with respect to the sub-scanning direction.15. The image forming apparatus as claimed in claim 14, wherein theoptical axis adjusting part adjusts the beam spot positions of theoptical beam with respect to the sub-scanning direction based on thedifference detected by the position difference detecting part duringwriting of an image.
 16. An optical scanner for scanning an imagecarrier, comprising: an illuminant part emitting an optical beam; adeflecting part deflecting the optical beam; an image forming partfocusing the deflected optical beam on the image carrier; and an opticalaxis adjusting part being provided between the illuminant part and thedeflecting part, said optical axis adjusting part adjusting a writingstart position of the optical beam on the image carrier with respect toa sub-scanning direction.
 17. The optical scanner as claimed in claim16, wherein the optical axis adjusting part comprises a movable mirror.18. The optical scanner as claimed in claim 17, further comprising anoptical beam detecting part detecting a position of the optical beamwith respect to a main scanning direction.
 19. The optical scanner asclaimed in claim 18, further comprising a housing integrallyaccommodating the illuminant part, the deflecting part, the imageforming part, and the optical beam detecting part.
 20. The opticalscanner as claimed in claim 19, wherein the optical beam detecting partis disposed at a position in the housing toward a main scanning end froma position detected by the optical beam detecting part and said detectedposition is allowed to conduct free expansion relative to the positionof the optical beam detecting part.
 21. An image forming apparatus,comprising: a plurality of image carriers; an optical scanner forminglatent images on the plurality of image carriers, said optical scannercomprising: an illuminant part comprising a plurality of illuminants,said plurality of illuminants emitting optical beams; a deflecting partdeflecting the optical beams; an image forming part focusing thedeflected optical beams on the plurality of image carriers; and anoptical axis adjusting part being provided between the illuminant partand the deflecting part, said optical axis adjusting part adjustingwriting start positions of the optical beams on the plurality of imagecarriers with respect to a sub-scanning direction; a developing partdeveloping the latent images so as to form visible images; and atransferred member onto which the visible images are transferred fromthe plurality of image carriers, wherein the optical axis adjusting partcomprises a movable mirror and adjusts the writing start positions ofthe optical beams on the plurality of image carriers with respect to thesub-scanning direction.
 22. The image forming apparatus as claimed inclaim 21, further comprising a resist position difference detecting partdetecting a difference between the writing start positions of theoptical beams on the plurality of image carriers with respect to thesub-scanning direction.
 23. The image forming apparatus as claimed inclaim 22, wherein the optical axis adjusting part is controlled throughfeedback based on the difference between the writing start positionsdetected by the resist position difference detecting part so as toadjust the writing start positions of the optical beams.
 24. The imageforming apparatus as claimed in claim 22, wherein a distance betweeneach of transferring positions of the latent images onto the transferredmember and a detecting position of the resist position differencedetecting part is set as an approximately integer multiple of acircumferential length of a driving roller for driving the transferredmember.
 25. The optical scanner as claimed in claim 16, wherein theoptical axis adjusting part comprises a wedge-shaped prism.
 26. Theoptical scanner as claimed in claim 25, wherein the optical axisadjusting part adjusts the writing start position of the optical beamwith respect to the sub-scanning direction by rotating the wedge-shapedprism approximately on an optical axis.
 27. An image forming apparatus,comprising: a plurality of image carriers; an optical scanner forminglatent images on the plurality of image carriers, said optical scannercomprising: an illuminant part comprising a plurality of illuminants,said plurality of illuminants emitting optical beams; a deflecting partdeflecting the optical beams; an image forming part focusing thedeflected optical beams on the plurality of image carriers; and anoptical axis adjusting part being provided between the illuminant partand the deflecting part, said optical axis adjusting part adjustingwriting start positions of the optical beams on the plurality of imagecarriers with respect to a sub-scanning direction; a developing partdeveloping the latent images so as to form visible images; and atransferred member onto which the visible images are transferred fromthe plurality of image carriers, wherein the optical axis adjusting partcomprises a wedge-shaped prism and adjusts the writing start positionsof the optical beams with respect to the sub-scanning direction byrotating the wedge-shaped prism approximately on an optical axis. 28.The image forming apparatus as claimed in claim 27, further comprising aresist position difference detecting part detecting a difference betweenthe writing start positions of the optical beams on the plurality ofimage carriers with respect to the sub-scanning direction.
 29. The imageforming apparatus as claimed in claim 28, wherein the optical axisadjusting part is controlled through feedback based on the differencebetween the writing start positions detected by the resist positiondifference detecting part so as to adjust the writing start positions ofthe optical beams.
 30. The image forming apparatus as claimed in claim27, wherein the plurality of image carriers comprise just four imagecarriers corresponding to four colors: black, yellow, magenta and cyan,that are arranged in a tandem fashion, one of said four colors ispredetermined as a reference color, and the optical axis adjusting parthas three wedge shaped prisms for adjusting writing start positions ofoptical beams for scanning three image carriers other than the referencecolor such that said writing start positions coincide with a writingstart position of an optical beam for scanning an image carrier for thereference color.